I just replaced the reservoir cap couple of weeks ago. Thanks for your suggestions guys. I will check the heater core.
I take the car to the track once a year and a few RAC days as well which is one of the reasons for colder thermostat. I didnt think the level of coolant would be dropping for weeks as the air escaped, but it might be a cause.
Any suggestions on where to get a compression testing kit from?
Thanks again guys,

Adi