Hey guys, I need a little help.

Iv got a Ford Focus 1.8 TDCI (03). Recently (Felt it more now its cold), my heating went from being hot, to luke warm. The temperature gauge went up and down like a yoyo, but never overheated. I changed the thermostat, as I thought this would cure the problem.
Now, changing the stat has cured the temp gauge going like a yoyo, but I still only have slighlty warm air coming through. Iv put new coolant in, bled the system, with no result. Also, when I start the car in the morning, the engine fans are constantly on. The vehicle is at normal operating temperature after about 6-7 miles of ordinary driving (60mph).

Am I missing something obvious, or is this a common problem with the diesel engined fords?

Also, is there a gasket on the sump as mine leaks, but am unsure if its just silicone, or if there is a gasket I can buy?

Thanks guys, Ash.