I was woken at 4am by the worst noise - it turned out to be my boiler. I turned it off, reset it, then turned on again. I guess it had been running for about 15 min before it first went, judging by the temp of the rads. It fired up, ran for about fifteen secs, then made knocking sounds, then cut out then made the loudest gurgling sound like gallons of water swishing around the very top part of the boiler by the flue. Its a combi boiler, about ten years old i guess, was serviced a couple of years ago. Ill bleed the rads when i get in from work but what else could it be? Its not the water pressure = thats fine, its on 1.5 bar.