Problem with Britoney Combi 80?

I have this boiler in my caravan , we can get hot water no problem but when I turn dial to radiators it just wont come on , sometimes it comes on and works for a short time then cuts off & wont come back on. I thought it was a problem with the clock timer but even when I press the override button it does not come on , also the pressure bar is at 0 how do I raise this as I think it should be higher.