Joss, thanks. thats a bold diagnosis. Is it easy to replace the thermostat in JTDs?
Yep reletively simple but if you car runs above the marker left of 90 on the temp gauge then yours is working correctly. If you do want to change the stat. The method is as follows.
1. Undertray off (you'll need torx to get it off)
2. Take header tank cap off.
3. Undo lower rad hose and let water out. (passenger side)
4. Undo engine cover (3x 10m)
5. Under upper turbo hose (screwdriver)
6. Take of battery connections (10mm)
7. Undo battery clamp (13mm)
8. Lift out battery
9. Undo all three hoses connected to the stat
10. unplug temp sender plug
11. undo 2 6mm allen bolts holding stat on.
12. Have a cup of tea before reassmbling
NB. There is a bleed screw on the top of the rad on the back on the left hand side as you look at the front of the car. (plastic) undo this before filling the header tank back up. Helps fill the rad and get air out of teh system. When it comes out the top of the rad put the bleed screw back in. Run teh car up with blowers on full with hight heat and header tank cap off. Fill then put cap on. Go for a drive. Check level next day and top up accordingly