Just make sure you bleed the system. I know it's not as big a problem on Minis as it is on other cars but it can still cause a problem. Check real close for small coolent leaks. When I worked on Fiats I had seen small pin holes couse the system to get air bubbles in them. The owner would not see a puddle on the floor and when the coolent was checked the level would look correct.
Check for coolent stains around the water pump, all connections and gaskets.
Hope you get it worked out.
