Whoever makes this for British Gas, the fault sounds like the diverter valve stuck.
If you mean the system pressure is low, then this needs to be topped up via the filling loop or link to just over 1 Bar cold.
No doubt your hot tap water is not as hot as it used to be?
Need to get bg in if its still under warranty, or at least a CORGI to inspect/repair.
barry boiler, December 2009