I had my thermostat replaced but the old one was clogged with gunk and when cleaned seems okay, the new one was fitted anyway. I had the car serviced at main MGR dealer every 15000 miles which I believe should have meant the coolant should have been flushed and changed but obviously wasn't. (100K miles plus).My point is that it may be worth flushing the cooling system before attempting this awkward task. I shiuld say that at 165,000 she is still going like a dream and getting 50+ mpg on journey to work each day. Mostly dual carriageway and motorway but it was nice to have a warm car again.
John