Lou
my mate Simon had exactly the same problems recently
fitted a new thermostat , 13 quid from ashburton . still the same so they did something to the stat , took something out
there was still an air lock there but they managed to get it out , eventually ,
now, he's rang me this morning(10 days later) , his head gasket HAS gone
always leads to the same thing
the thing about gudge etc is only if it's been gone for a while , it doesn't always show up especially if it's recent .