ICOS HE15- Boiler ignition banging?

Have Icos HE15 boiler. 2+ years old. Regularly serviced. Works OK on heating + hot water but has a problem with hot water only. On hot water the boiler cuts in/out on thermostat as hot water tank temperature stabilises. Sometimes after boiler has been off for 5-10 minutes and heat is required for hot water, the fan starts and ignition sequence commences but there is a boom as gas ignites.
There are no fault messages on LCD display.
Any suggestions