changing the thermostat - how to

Looks like I may have to..thanks DAVID. Is it possible to remove the valve ?

The jiggle valve is contained within the plastic reducer that connects the offending spaghetti pipe to the small bore pipe across the engine to the expansion tank.
The valve shouldn't be an issue, but you could remove the internals, yes.

Thank you for all you help and advise

Correction "The thermostat housing is bolted to the engine with a single 10mm bolt, "

The one on my Mk 1 mpi F was held on with an 8mm bolt.

So dont be put off if you cant find a 10mm bolt, if it's a 8mm on yours it still definietly needs to come out