Why isn't Remeha Avanta Plus 28 Combi Boiler working properly?

We have a Remeha Avanta Plus 28 Combi Boiler which has been working fine for years. Yesterday we had a plumber in to move some water pipes as part of a bathroom re-furb and he removed the bathroom rad. Nothing else on the boiler was touched but the heating doesn't now work, only the hot water. The signal from the stat is being received, pressure is good and there are no error codes. On receiving signal the boiler fires up but only for about a minute, then seems to run on quarter power - very weak plume of exhaust (in cold air) from the flue.