hot engine

hi, ive just installed a new head gasket on my defender 300 tdi after not paying attention to the temp gauge while towing. i went on a 100 mile round trip yesterday , temp was fine all the way there . the needle was dead in the centre of the gauge. however the last 20 miles the gauge was heading to wards the red. i thing i still have the same problem . i was watching the gauge like a hawk this time as we all know the expense . any one with the same exprience or trouble shooting ideas.
cheers

Check your thermostat is opening. Are the radiator hoses hot to the touch once the engine is up to temperature? If not you either have no water in the system or your stat is stuck closed.

Or a cracked head...eek!