Right , after nearly 10 years of driving occasionally and the heater only just registering at the top of the blue on the gauge I decided to sort it out. Knowing that the EGR thermostat had failed I replaced this with only marginal heat gains. I then went down the route of the Renault 'in-hose' thermostat for the top heater hose , but due to being unable to fit it without removing the engine side of the hose decided on another option . I purchased the Rover 75 tdcdi top hose modification which allows access to remove and change the thermostat housed within. Well , the outlet part fits inside the top hose perfectly (after cutting the top coolant hose to fit inline) but the inlet side from the engine was an absolute pig to do ! Anyway , all fitted and the car warms up to the 12 'o' clock position and the heat is fantastic. However , if I give it 'some beans' and or turn the heater down the gauge goes up into the red with the warning of high coolant temperature . Turning the heaters on full again and to max blow it drops back to the 12 'o' clock position. Now in the current temperatures I can live with this for a while albeit arriving feeling like a roasted chicken , I will not be able to drive in summer without fitting my other new top hose and removing the inline thermostat with the increase in fuel consumption and potential engine wear. I am thinking along the lines of a rad flush , does anyone else have any other recommendations ?

If I remember correctly, when the coolant temperature overheats the ac condenser fan starts to increase the airflow over the radiator and thus cool the coolant down.
Might be worth checking this when the coolant is overheating.
Having had a Td6 for the past 10 years I have to wonder how your coolant is overheating, at this time of year if anything I would half expect the coolant to eventually get up to temperature but not a lot more.
Some people run these cars without radiator fans apparently without issue.
If this situation has only come about since your thermostat modification I would be inclined to suggest this is the source of your problem, whatever the reason.

I pulled in to the drive the other night after work and my other half thought there was a jet flying over the house = it was the twin air con fans going full chat ! Think I need to try and get access to the radiator and feel if there is a temperatre difference between the top and bottom and see if the rad needs a flush . likewise I am aware of the plastic fan BMW water pumps for coolant circulation !

One thing less then.

Just thinking out loud ...

Is your thermostat housing up near the radiator? If so, the thermostat is at the end of a long hose of water that is not flowing anywhere, so possibly the hot water in the engine is not getting to the thermostat hot enough to open it? Compare this with the mod where the thermostat is at the engine end of the radiator hose - the thermostat is right in the water flow from the engine outlet and so will react quickly to changes in water temperature.

If you get the engine to the point where it is overheating, can you check how hot the thermostat housing actually is? Might a kettle or two of boiling water over the housing and the hose near it ensure it is hot enough for the thermostat to open and see what happens?

I'm going to state the obvious and check that the thermostat is fitted the right way round? Definitely no airlock in the top hose?

I done the mod fitting the thermostat in the engine end of the hose and it was easier that I thought it was going to be cutting the original hose clip off with a Dremel and fitting a new jubilee clip.

stuart_2006 read your write up of the Renault thermostat mod and noticed that your thermostat is next to the engine outlet and not the inlet to the radiator , is that correct ? I also purchased the Renault mod and a new genuine top heater hose so think I will fit it but next to the engine outlet this time. Thanks for your input suspect it could be the answer as currently thermostat too far from the engine outlet

Yep, mine is next to the engine outlet.

As you bought a new heater hose (I did as well), you'll need to remove the plastic connector from the end of that at the engine end. So that gives you a chance to practise removing the hose clip with the Dremel. Removing the bolt holding the dipstick tube in place gives you a little work room to work.

After connecting the new hose at the engine end, I filled the hose with coolant as far as possible before connecting at the radiator end - wasn't sure how easily the air in the hose would bleed while the engine is cold and the thermostat shut.

Fitted this morning and all topped up with coolant and bled using the 3 bleed screws . Will give it a try tomorrow. I did test the thermostat in the hose before I fitted it to prove it operates correctly .

Many thanks for your input stuart_2006 , the issue was that the thermostat was too far from the engine. All fine now with the needle at the 12 'o' clock position whilst giving it some beans and all the heaters blowing cold Only issue now is the front exhaust clamp that clamps the turbo to the exhaust went awaol ! Anyone any ideas re size or part number ?

Can I suggest getting a cheap infrared temperature gun. Just point the red dot in any inaccessible part of the engine that you can't reach. You could see if a thermostat is stuck closed or open on either side. Used mine to find a sticky brake caliper.