You need a fan cowl. Our 2a had a Series 3 rad in it with a fan cowl and the additional cowl you can get. You could run it all day without getting hot. A capillary gauge with numbers is excellent in these old motors to keep an eye on temps. The only time it got near to being hot is when we had...
It will run with the engine running if you bash the heater box,, poor connection inside most likely.
I will try a separate earth at some point before the heater box comes out.
As usual the problem is in the least accessible part, inside the heater box.
I used wire wiggling and bashing things as part of my diagnosis. Bash the heater box and it will work.
Unfortunately 200tdi have been taking a hit recently. They haven't been under £10k for a long time, seen quite a few now at a fair bit less than that recently.
Have a look on Ebay for a rough guide.