Ravenheat CSI 85T not firing?

Our Ravenheat boiler has given up on CH and HW. There has been an intermittent fault (fault described below) for a few months but after resetting the boiler it would work as normal. It has progresively got worse and now it refuses to work at all. If there is a demand for HW or CH the pump starts and the fan kicks in. After a few seconds the overheat and/or flame failure indicator light comes on. I would like to call an engineer out but was made redundant and have no money! I am a confident and proficient DIYer and if it is a relatively simple problem would like to fix it mysely. Can anyone help?