Cheers guys tried your suggestions and it now confirms the leak is somewhere in the system. As I cant find the leak under the accessable floorboards I think I may try and get the system re-plumbed rather than try to find all the pipes some of which are running down walls and under a concrete...
I have had the BAXI Duotec Combi installed around 3 months with no problems, but over the last 10 days the water pressure has been consistantly reducing so that the HW & CH stop working around every 8 hours and I have to re-pressurise. :cry:
I realise this is likely to be due to a leak...