I have the head off the 2l Pinto at present and the area around the thermostat housing seal looks a bit iffy.

Its probably not as bad as it first looks as the gasket edge sits about 3mm back from the actual hole and indeed it was not leaking before.

I will use red Hermatite with the standard (paper) gasket, but is there anything more I can do while the head is off?

A previous owner has used cheap methanol blend antifreeze. No worries as long as the gasket has a complete ring to seal on, if you wanted you could use grey or black Loctite RTV to for the seal or make the surface good with metallised filler.

odd to see this as im just changing the thermostat gasket on my pinto. I know fully well why it leaked though - when fitting the new engine i removed the thermostat housing to give more clearance at the front as i was lowering it into the engine bay.

I should have just unbolted the headlight bar, i ended up doing it anyway.
I should have been patient and ordered a new gasket instead of being impatient and just refitting it with Hylomar.

I'd use the grey Dirko Seal on it over the Hermatite, it's great stuff, a lot of modern engines have the sump, oil pump, water pump etc fitted up just with this, even if you fit a gasket a smear of this either side won't go wrong.

Is there an option to buy a thicker slightly more complient gasket?

You really don't need anything special either use a paper gasket or a decent quality RTV follow the instructions and let the bead of sealant start to cure before fully tightening.

If you want to use a paper gasket and jointing compound Red Hermetite is old fashioned and messy 1950s era stuff use Blue Hylomar.