Just bought a freestyle 51 plate. Coolant kept disapearing & car over heating but no visable leaks. Tracked it down to a pipe blocked with air running from engine to radiator. Blew down it, coolant came gurgling through. Driven it to work today and its done it again. Could it be a faulty thermostate or as my wife fears a cracked head gasget leaking air into the system? Thanks

In my limited experience, head gasket failure usually displays other symptoms:-

Water in the oil, showing as sludge in the rocker cover and oil filler cap.

Oil in the water, showing as a greasy black coolant expansion tank which should be white and transparent.

Excessive white steam from the exhaust, even with the engine hot.

Coolant loss can be as simple as a faulty radiator cap releasing steam at too low pressure which can easily go unnoticed and leave no trace of a leak. It is unlikely to be a faulty thermostat as this would usually fail open although you can test this by removing it altogether and see if the fault remains..

It would be safest to get the cooling system pressure tested which should show where the fault lies.