- The cap seems to work, as when you release it you get some pressure release (much less when fully cold)

There should be no pressure in the system when the engine is fully cold. Possibly indicates head gasket issues starting.

When the engine is cold, I squeezed the coolant pipe going into the thermostat housing and it wasn't hard - so the system isn't remaining pressurised (cylinder head gasket?)

When the system is cold there is little vapour pressure so the pipes will be squeezable. The source of the pressure is the heated coolant.

The tank level is at the Min when cold, and roughly halfway between Min and Max when hot

Perfectly normal. Antifreeze expands considerably when warm-much more so than plain water.

Look for some crusty residue around hoses (especially at the ends), thermostat housing and radiator. My bet is that coolant will force its way through a poorly sealed hose when the coolant is very hot and under pressure. This normally occurs when the coolant flow quickly decreases while the cylinder head is well over 100C such as when you come off the motorway and have to stop at lights or a roundabout.

You won't see this happen if you've been pottering along suburban roads for a couple of miles before you get home and pop the bonnet open.

I once switched the engine off immediately after a hard hill climb and the temperature gauge climbed into the red. Looks alarming, but the head had been at that temperature all along.

Relax! Your h/g is fine!