I have a two and a hlf year old 'Thema F23E' boiler which works fine for HW, but when you try to get CH to work it fires up for about 5 seconds and cuts out completely. The CORGI plumber that installed it for me flushed the system as he thought that the pipes were blocked but this has had no effect, and he has basically washed his hands of it. Can anyone offer any advice please?