Glowworm (British Gas) 330 boiler constantly cycling on and off?

I have a glowworm (British Gas) 330 boiler which was already installed in my new house. It works fine with the hot water however when it switches to heating only it keeps getting hot too quick and clicking off over and over. My heating system has a new reasonably powerful pump on speed three and runs in series on 15mm pipe. Please could someone who knows about this let me knoe what my problem is. Many thanks, Martin