My Disco 300tdi's heater has recently been blowing cold air even when set to hot, and then after a while it might start to blow out warm air. The temp gauge for the engine has been reading normally for town driving but I took it on the motorway and I hadn't even got one junction down before the temp indicator was nearly at the red. I was came off at the next junction and before I had reached the end of the slip road it was back to normal temp. Day after car was fine on town driving. Went on the motorway today for the first time since the hot reading and it was very quickly straight back up to the red so i pulled off the motorway. It did cool down quite quickly over a few minutes, but not instantly as before so I allowed the engine to cool back to normal temp before returning home via standard 30/40mph roads at which the gauge stayed normal....

Also sometimes when driving along recently I have heard a gurgling type noise? or like liquid sloshing around somewhere?

I'll be taking it to the garage tomorrow but just wondering what kind of problem and bill I can expect?

Thanks.

Just a thought but sounds like intermittent thermostat problem. Easy to check and fix.
Just feel the big top hose out of the engine, it will be cold after just starting but should warm up once the engine is hot. If the engine is hot(on temp guage) and the hose is cold the thermostat is not working. Best to just change it and rule it out they are cheap and easy to change, its under the aluminium elbow that the big hose attaches to. Make sure you fit a new seal/gasket.

when my pulley fell of my alternator losing me drive to my water pump (power steering alternator etc) i had the same issue i thought that faster speeds would cool the engine by airflow however it just got hotter until i stopped or slowed down i put this down to the water pump losing drive i guess you would know if you've lost your belt but maybe your water pump is nackered they aren't expensive and are quite an easy fix although the above answer is just as plausible