Thanks for your reply, funny you should say that I took it to an apparent mg specialist garage, which I shall not name, and said to them exactly that - I don't think my rad is good. They gave the classic 'it's yer 'edgasket mate', which was somewhat surprising as I used them for work in the past and they proved to be pretty good. The irony is these guys changed my underfloor coolant pipes about 4 months ago and didn't bleed the system from all three points. They only bled from two because apparently one is a 'pain in the arse' to get to.
By the way yes that's exactly it - the fan cuts in but doesn't seem to be able to bring the temperature down.
'Pain in the arse to get to'? Yes, one has to take off the engine cover to get to one of the three but such is life. For a MG specialist not to be bothered is a disgrace. What else cannot they be bothered doing properly...? Stop using them is my advice!