(posting in case other people find this via Google - clearly Mark will have fixed this by now or frozen to death)
The display should give you a fault code, and you can look this up on the web - the full list of fault codes is out there somewhere. I found them a year ago, but they've moved - sorry - otherwise, I'd post a link.
The fault that Nik Kay is referring to shows up as fault code 72 on my boiler, which is something like "low pressure on return side". My manual says to keep the pressure between 1 and 1.5 bar, but omits to say this is the *cold* pressure, and there's a big difference between cold and hot pressures - Nik is probably right that it needs to be more like 2-2.5 bar when hot and operating.
Steev Wilcox, December 2008