Hi all here is some information.
Had the usual probs of overheating after trial and error. It is now sorted and running as it should.

The main culprit all we all know was the metal pipe where the pressure cap goes.

To check if the cooling system is working properly Fill the as recomened in the manual. Make sure you bleed the heater tank as in the manual.

Make sure the expansion tank level is between the max / min Note the level. Then go for a run.

What should happen as the pressure increases the water will be pushed into the expansion tank. After cooling and the pressure drops the water is pulled back into the system. It has to return back near to the same level. Lower no problems.
If the water level is higher then air has been sucked back into the system which causes the air lock. Then leads to the overheating.

Places to check. Rad Cap. Bleed cap on the heater tank. But the main culprit as we all know is where the pressure cap fits. If there is a pinhole gap it will suck air than water. Where it seals it must be 100% airtight.

Mine was rusty and very rough. even smoothing it off made no difference.
So tried to get a rplacement. New £150 Arghhhhhh!
But was then told they no longer have it in stock. Then I found a breakers called

Used Hijet Parts (Jeremy Thorne 01225 344481 mob 07734604021. Sent the pipe after he checked it over £20 posted. Brilliant.

I also got both trackrods with the ends because mine had siezed to could not track the wheels Cost £20 posted.

Hope this helps anyone with the same problems
Also cured a leaking sunroof. Taking the sunroof out is easy