Heater/Engine Temp

Now firstly, I have read through a bunch of old threads, so I'm half way there.

My 97 Disco 1 300TDI seems slow to warm up, it does after about 12-15 mins get up to normal temp, then, sometime after (maybe 20 mins at "normal" running speed - 40-60mph) the heater will work, at which point the temp guage will drop right down. This last bit is the bot I haven't seen on the other threads, unless I missed it!

So, I guess I'll swap the thermostat, but will that sort the last bit out or is there something more sinister afoot?

Cheers,
Steven

Change stat, make sure its properly bled of air and make sure your rad core isn't restricted