Thank you all for your replies, I appreciate it. A friend suggested letting half a bucket of water out of the coolest radiator, while the pump was turned on, as he suggested there was an airlock in that rad. So I opened the bleed hole and let an amount of water out, and could hear the header tank trickling water back in. It hasn't really helped but what I did notice is that the water was brown, and didn't appear to have any anti freeze in it. So I'll try the draining of the system and filling with a 25% mix. For someone who has lno experience of this, what is the best way of draining the system (without flooding the carpet!) and once I've removed the rads and flushed out and re-attached, do I fill the system again by pouring the 25/75 mix directly into the header tank, and how will I know when it's sufficiently full?