Discovery TD5 heater problems

Disco td5 Heater not geting hot, so changed thermostat still not geting hot.
Can any one sugest what the problem might be.
Thanks
Mark

silly question coming up, is the antifreeze in the vehicle upto the required strength, have you lost water recently,easy way to check if its the heater or main engine are the pipes feeding the heater when the engine is at normal temp hot or cold, and also check the heater controls inside the vehicle, not suree what they look like are they knobs you turn or levers you slide left to right

I fancy it it could be the flaps in the car which divert the air to hot or cold, i think there is no on off valve as such

dont want to worie you but mine did that turned out to be head gasget onw been done twice in three months and back in again with suspect head craking