Thanks for the replies chaps - and on the weekend too!!
It's a Potterton Suprima 50 and was installed by British Gas just over 10 years ago. It's covered by a BG maintenance contract so has been checked/serviced every year. Apart from needing a replacement main control board back in 2005...
I've just put X400 in my CH system and will be flushing this out in a few weeks.
I live in an area with very hard water so I have a water softener which supplies virtually all cold water outlets in the house. Currently this includes the supply to the loft header tank for the CH system (it's...