Can I replace my old Potterton flamingo 50?

you can get adaptors to fit most pipe sizes for metric to imperial connections, get a second opinion.
it may be worthwhile considering a full re-pipe though depending on the system design and to get the most out of your new boiler, and modern radiators are better at chucking the heat into the room.
at the very least have some type of flush carried out on the system, 30 years is a long time for sludge to build up.

Ian
April 2010