Why does my boiler ignite with a loud bang?

I have an Ideal Icos HE12. There is heating and hot water, but for some time now there is a very loud bang when it ingnites. The bang doesn't always happen, but increases significantly when the system is hot already. Initial ignition from cool seems fine. Now I cannot set the room thermostat temperatures above 20C. It's almost like there is no recognition of the target room temperature and ignition keeps coming on and cutting out in short intervals of 3-4 minutes.