how to fix my BAXI INSTANT 105E?

system is 4 years old. problem is the heating will not come on, i only get hot water. main boiler pilot light comes on when switching system on from cold, system temperature climbs to 80 degrees on the flame failure neon after about a minute it cuts out. flow pipe is hot about a foot from the bolier and that is it!
recently had diverter valve replaced as corrosion in one of the threaded joints caused a leak.