Potterton Suprima 60L no heating but hot water ok!?

Recently my Potterton Suprima 60L started playing up and has a red fault light displayed. It won't reset and have even tried switching off for long periods. It 'tries' to work when demand for heating, fan starts, relay klunks in, pilot light fires up...then shutdown and fault light. Yet, run a bath, have a shower or leave the hot tap running for a long period it fires up fine supplying hot water. Does anyone know what the problem may be?