Is that a Potterton Netaheat Electronic?
If so, I recomend you get rid of it as soon as possible, there's a risk of explosion and gas leaks, read this: (http://baldwin.butterworths.co.uk/search/sql/dataitem.asp?id=10975&tid=7)
Also, you should have a wall mounted control panel, with the options (CH = Central Heating and HW = Hot Water) make sure they are adjusted properly, other than that, it seams, something else is really wrong, you may not be able to fix it without expert help which I can't provide because I don't know.
MrAnon
March 2005