I had the same thing on my 200,heater would not get hot,water levels all seemed ok, engine temp was all ok,took entire thing to pieces ond flushed,still nothing,put new thermostate in ,didn't do anything apart from cost money as one of the bolts had siezed so had to replace housing as well, what I did notice was it was giving the effect of an air lock which shouldn't happen.Anyway it turnded out to be one of the header tank breathers, they have a restricter in which had corroded up ,it was the one that comes of the thermostate housing.If you feel the pipe to find where it is then cut the pipe,pop it out and clean it ,then put it back in and put a clip round it it should be ok.
