Tackled the problem today, I mixed up a bit of JB Weld and applied it to the end of the steam pipe, after giving the pipe and thermostat housing a good scuff with some 120 grit sandpaper to give the JB weld something to bite into. Stuck it into the thermostat housing and gave it a few gentle love taps with a deadblow and block of wood to make sure it was fully seated. Going to leave it overnight to cure and come back to it tomorrow afternoon, see if it holds up. if it does, great, if it doesn't, I'll be drilling and tapping it, and putting a threaded fitting in there. Also need to wash the engine bay down now to remove all that sprayed and dried coolant residue. Good to see the ASA distributor cap seal can resist a direct spray of hot engine coolant though!