Thanks Mark, I did but not convinced it is any good. I think I will take the expansion tank off and send it to someone to re-make within the dimensions and start with a fresh cap to their spec etc.

Spec r seems the obvious choice as long as it doesn't take too long. I will rig up the standard Sierra plastic one I have for now and use the std cap to see what that does.

You are 100% right about that turbo, you can be pootling along in 3rd
or 4th and it just seems to spool up, if I hold it around 3000 rpm cruising and then move my foot a bit the thing just takes off from not a lot to big boost.