Rover 45 Thermostat Problems

hi, i got heater problem recently. when i was driving at motorway, my heater temp going up and heater doest work, it's going cold. I suspect it was because of waterpump but i've already replaced my waterpump last year.

Dec 23rd I lost alot of water through rat bitten rad.hoses(incl.main top hose) had replaced and since then for the 1st time since Ive had the car, the temp gauge starts to rise after about 1 mile, if heater and fan switched on, drops and s...