An update - the garage thinks they've found the problem - a bad coolant temperature sensor. They kept it overnight and it started up in the cold this morning. I don't think they are completely sure it is the problem though, so...
I'm going to put in some meths later today and try that as well - will let you know how I get on
I mentioned the water problem to the garage and they said it wouldn't be a problem unless tank was a 1/4 empty as the fuel is taken from the top of the tank. I think with these cars the fuel is taken from the bottom though?