Glowworm Swiftflow 80 not heating?

7 Year old glowworm swiftflow 80 (honeywell) has recently got a bit noisy and last week, out of the blue stopped heating either when the hot water is on or when the central heating is on. It sounded like a classic fan issue, so I've changed the fan for a new one. It worked straight away for about 5 minutes, but then stopped again. It came on again for 5 minutes after about 15 minutes of being off, and as of now comes on every now and again for the heating, but not as it should, but never for the hot water. Can anyone help from here. When the system has been off for a while, if a open up the hot water, i get scalding hot water for a second or two but then straight back to cold, even when the main nurner has not been on. The pilot light always stays on and never goes out.