I am sorry to tell you that your battery is stuffed.
It is likely that broken connection stopped it charging and caused the problem. But once a battery goes that low it will never be good again. It may show good voltage , around 12.6, but i doubt it will get to 12.8v again, after standing of course, but it will never have the same cpavity again.

This issue has only came about over the last 2/3 days and Ive had the battery tested and its ok so I'm hoping the issue is the broken charging circuit.

Soon find out. Range Rover quotes.....

The Luxury That�s More Than a Luxury.
Conquer the world, and look good doing it.
Built for the unexpected.
Over prepared, but never underdressed.

2007 Vogue SE TDV8 Epsom Green (old car).
2010 Discovery 3.0L GS Mk4 Black.

A good battery sits at 12.8 volts fully charged, thats 24 hrs after last charge, 12.6 volts is 80% charged and thats 24 hrs after last charge.
If you are getting 12.6 under charge then one of the two is less than ideal.
But each to his own, i personally would replace the battery as soon as i confirmed the alternator was good.
Modern cars do not like low battery voltage when cranking.

my battery was so flat I could not open doors even the clock stopped. after she spat out a spark plug she sat for over a week stone dead gave her a jump after repair. battery charged up quickly and has not had any problems since. final stage resistor is common problem with flat battery worth changing not expensive . I have top of range battery in mine but she will still flatten battery if interior lights and cd player on any length of time hope this helps

That fuse is aftermarket and shouldn't be there.. suspect that an over cautious previous owner put it there to stop it bursting into flames... but that's the under seat wiring.. and well under 30 amps.. so it wouldn't have worked anyway. Just connect it directly to the battery, I'm surprised that it hasn't given issues before as the starter can draw up to 480 amps.. Maybe it had a winch at some point.. even then it would have to be a small one

I'm surprised that it started as that lead if I'm not mistaken goes through the bulkhead to the starter motor.. Anyway ignor all this you've only got 12.469954356 volts so your battery is fubar'd, I ran for 18 month with a battery sitting at 11.5... it was only when the DSP drowned and decided to take the battery with it that I had to change it, up til then it would go 2 weeks without starting and still work first turn of the key.

So connect it direct and see what happens, just don't be shocked if it doesn't hold a charge, but be pleasantly surprised if it does.. And booster pack or jump leads might be a good idea for a few weeks  Yesterday I couldn't spell Engineer... Today I are one!
Inventor of the 'Guide-o-Matic automatic wheel alignment tool'
Former long term L322 owner, Up/Down graded to a Classic Tractor!

I've had it charging and now its fully charged and multimeter says its 12.8v so now I'm going to start it and turn everything on and see what's at the batter so I know if the alternator is working.

I have connected that cable direct back to the pole and when its not connected the car still starts so I think its not coming from the starter but the alternator cos without it the volts don't increase when everything is running. Range Rover quotes.....

The Luxury That�s More Than a Luxury.
Conquer the world, and look good doing it.
Built for the unexpected.
Over prepared, but never underdressed.

2007 Vogue SE TDV8 Epsom Green (old car).
2010 Discovery 3.0L GS Mk4 Black.

Have a feeling that wire connects to the bottom of the explosive pack this is a pic from my 2007 can't see it properly but might be worth checking.

Click image to enlarge

Well battery charged at 12.8v, when car is started the volts drops and when everything's on the voltage drops to 10 or so volts.

I have just checked the battery again after the car has been sat idle and the battery is reading 12.5v so I'll check tomorrow and see if it drops overnight.

Last time I checked it was going up to 13.8v or so running with say heated windows on but its not even doing that now. Range Rover quotes.....

The Luxury That�s More Than a Luxury.
Conquer the world, and look good doing it.
Built for the unexpected.
Over prepared, but never underdressed.

2007 Vogue SE TDV8 Epsom Green (old car).
2010 Discovery 3.0L GS Mk4 Black.

That don't sound quite right, with the engine running and possibly having blipped the throttle to excite the alternator into life the battery should be reading around 14v ... even with everything on it should still be above 13 volts, I'd expect to see 13.5 +... you might have an issue with the alternator

And I think that I might have sussed what has happened to the battery lead.. if the car was in a prang and it triggered the airbags, it will also have blown the battery connection, some bodge merchant has replaced the short lead with a normal nut and bolt job and placed a fuse in line because they didn't know what they were doing  Yesterday I couldn't spell Engineer... Today I are one!
Inventor of the 'Guide-o-Matic automatic wheel alignment tool'
Former long term L322 owner, Up/Down graded to a Classic Tractor!

That was my thinking miggit  There is nothing that can't be fixed with a hammer😜😜
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I was confused myself why the 300 amp fuse was there I'm still confused dot com what that lead supplies.

I charged the battery and tested the voltage and it was sat at 12.5v, tested last night and it was still at 12.5v and tested again today after 24hrs and its at 12.4v so I'm thinking the batter is holding its charge and its the alternator.

Will start it again, run it with a few things going and see if the alternator kicks in. Range Rover quotes.....

The Luxury That�s More Than a Luxury.
Conquer the world, and look good doing it.
Built for the unexpected.
Over prepared, but never underdressed.

2007 Vogue SE TDV8 Epsom Green (old car).
2010 Discovery 3.0L GS Mk4 Black.

Update..........

I started the car and started all fine, turned everything on and the voltage dropped to 10v and when revved went up to 12 or so volts.

Turned engine off and yep flat again.

So after all the above it was the battery so just changed it for a new one and the volts are now steady not dropping and the alternator is working fine.

Thanks guys all your help is much appreciated indeed. Range Rover quotes.....

The Luxury That�s More Than a Luxury.
Conquer the world, and look good doing it.
Built for the unexpected.
Over prepared, but never underdressed.

2007 Vogue SE TDV8 Epsom Green (old car).
2010 Discovery 3.0L GS Mk4 Black.

Still baffled where that above cable is going to. Range Rover quotes.....

The Luxury That�s More Than a Luxury.
Conquer the world, and look good doing it.
Built for the unexpected.
Over prepared, but never underdressed.

2007 Vogue SE TDV8 Epsom Green (old car).
2010 Discovery 3.0L GS Mk4 Black.

The cable that goes to through bulkhead, via the + cap jump starting point, is the main power feed to and from the engine..ie. it goes to the starter motor, and comes from the alternator. Yesterday I couldn't spell Engineer... Today I are one!
Inventor of the 'Guide-o-Matic automatic wheel alignment tool'
Former long term L322 owner, Up/Down graded to a Classic Tractor!

Yes but the one that had the fuse if its not connected the car starts as normal so I cant be coming from the starter so it must be going somewhere else. Range Rover quotes.....

The Luxury That�s More Than a Luxury.
Conquer the world, and look good doing it.
Built for the unexpected.
Over prepared, but never underdressed.

2007 Vogue SE TDV8 Epsom Green (old car).
2010 Discovery 3.0L GS Mk4 Black.