My Potterton Promax System 32 HE Plus no longer heats the house adaquately?

The other morning we had no HW or CH. When I looked at the Boiler I noticed the Pressure Gauge was below 0.5 bar. I let more water in and the boiler fired up. About 15mins later there was a loud knocking noise from the boiler and the pressure gauge was swinging erratically from the green to red section in tandem with the noise.
Having researched my Potterton manual I drained the primary circuit thinking it could be a blockage in the primary. This had no affect and the knocking persisted, until it eventually stopped.

Now the boiler takes ages to heat the house and is on all the time never seeming to reach it's set temperature. Although I have no reason to suspect a gas supply fault the flame from within the combustion chamber seems very tame.

Any clues as to what might be going on would be very much appreciated. Thanks