Got the Bastid bolt back in, top hose fitted - that did put up a bit of a fight.
Pressure tested the system and when it got to around 0.5 bar coolant pissed out from under the stat. :'( :'(
Its just had a new seal so god knows why its leaking.
Got to take it all apart for the third time, try and work out what the problem is, and then do it all again.
Maybe buy a new Stat housing in case this one has warped ?
Refill with just water in case it leaks again again,, you need to check that bolts aren't bottoming, threads are good in the housing, and it hasn't got hairline crack, also check the hose, they can leak, use a new jubilee
Cigar goes to BG.
I shall explain. Its a weird one. The bolts were indeed bottoming out. Had two identical but slightly shorter bolts in my stash and that appears to have fixed the problem.
Put it all back together (bastid bolt went in relatively easily this time) and pressure tested it to 1 bar. No leaks.
The reason its a weird one is the fact that those two bolts have been holding that stat housing in place for the last 6 years or more, and it only started leaking a couple of weeks ago straight after a coolant change.
Explanation ? I cant think of one.
Have had enough for today, so the fanimold, plenum etc will be a job for tomorrow, then fire it up, top up coolant etc.
But Im pretty confident its sorted now.