I've searched and learned that replacement of this is a 5 minute job but not sure and don't want to damage anything.

should the big black nut just turn by hand (mine doesn't) & is it a right hand thread? is there a special tool or way to grip it or just brute force?

Brute force!
It is a right hand thread, but more of a bayonet type twist. There's a knack to it but its really straightforward

When the new one is in place make sure the arrows line up

so when you say 'bayonet type twist' do you mean that it is pushed in and twisted like 1970's indicator bulbs?

it doesn't want to move!

Give it some brute force - it will move!

i cant get sufficient grip to turn it. needs a giant pair of mole grips or something similar

I wouldn't use mole grips. Put one of those oil filter strap tools round it and try and tease it off, once it gets going it slips off very easily.

Where are you dexion?

newcastle - it sounds like its so simple that once I've figured it out I'll be changing the filter every week just cos i can!

will try the oil filter strap tomorrow

Yes you will!!

Newcastle staffs or north east ??

proper Newcastle!

Why are you so eager to change it? They last about 25,000 miles?
Have you had some bad diesel or a problem?
Why not wait until your next service?

The Geordie one then!

the car is 2010 SE 50K miles - one owner with full dealer history. Since i bought it Ive fitted new disks etc all around and I need to fully service it too else I wont love it!.

yes the Geordie one!

Someone on here uses the phrase "if it ain't broke then don't try to fix it" !!
I'm guessing your fuel filter has been changed at the last service...
I'm also guessing your car runs nice and smooth - much similar to mine I guess!
As its a 2010 it may even still be in warranty at 50 k
I would leave it for another 10,000 miles and worry about it then!
If you are ever in my part of the world (over you next 10k) pop in and I will do it for you!