This is for a mate of mine. Two months ago he replaced his existing kaput programmer with a Horstmann Centaurplus C21. This is also now kaput. He was going to return it tomorrow but we've been having a read of the instructions. There is a link at the back of the C21 which should be left in place for FULLY PUMPED or CONTROLLED systems and removed for GRAVITY or PARTIALLY CONTROLLED systems. His system has a cylinder, a pump and a two port motorised valve, the pipes are all 22mm. We suspect that the hot water is a gravity circuit and the link should have been removed. Are we correct?