Here are better pictures of the Circoli thermostat that has burst after 36 hours on my son's car.
It looks like he has avoided any further damage and head gasket seems as if it is sound.
After fitting a new thermostat (not a Circoli this time) I have left the car running on the drive for an hour including holding at 2,000rpm until high speed fan kicked in and all seems well. I can't actually drive the car on the road as I am too expensive for his insurance so we will test drive later.
It does look like there is some sort of adhesive on the old one but it doesn't look like there is much. The new one had quite a bit of white sealant visible around that same joint and clearly continuous.
The car does not seem to be pressurising hoses, using water nor over heating so I can't see any reason except poor quality for this to have happened.
Original thermostat was changed because the car wasn't getting up to full temperature.
I hope this isn't due to head gasket over pressurising the system but there aren't any symptoms of that yet.......
Either way the expansion bottle pressure cap is 1.4 bar so this part should be OK for that (20psi) plus a very good safety margin.
DavidC: yes, buy cheap buy twice is true.........again. I really should know better!
This could have resulted in the need for me spending either a lot of time or money on this car. It might yet need time or money as at the end of the test drive (10 miles of fast road) it did need a top up of maybe a cup full of coolant. I hope that is the last of the air coming out and not the start of the car using water. No mayo in the oil filler nor excess water out of the exhaust so I will keep my fingers crossed. Very early yet to be confident there isn't damage waiting to show up.