200Tdi Defender. Head Gasket going? Advice needed pse?

I have a 200tdi Defender which appears to have considerable pressure in the cooling system. Cant see any sign of white emulsion gunk anywhere but I have noticed that on acceleration the temperature gauge goes up but returns to the middway position at lower engine speeds. Havent noticed any coolant loss or engine running problems. Seems a bit odd. Any ideas? Thanks. Mike

when you say considerable pressure, can you squash the top hose by hand? if head gasket leaking into water jacket you would be losing water through the exhaust.LEE