I've been using lots of coolant recently, about a pint every 200 miles. also constant pressure in the tank, even when cold and ihavent used it for a few days, so that when i take the cap off it pushes about 1'2 a pint back into the expansion tank. Been looking around and a lot of threads say this is probably head gasket. Any ideas?
Also to prepare the wallet for what might happen how much to replace the gasket?

Mine went last month. same thing, loosing coolant (no signs of leaks, header tank always under pressure etc etc) and then dumped it all over the drive while parked up over night. another sign was a gurrgle noise coming from the heater matrix on start up. Had mine done by a local Indi and it came to £800 including head pressure test