How can I fix my Isar HE30 boiler?

I can't get the central heating or hot water to come on. The display is blinking, first alternating with the letter 'H' and the number '9' and then after a few minutes alternating with the letter 'L' and the number '9'.
I've tried pressing reset, switched it off then on again and got the pressure back to 1 bar but still no good. When I reset the boiler it comes on as if it's going to start then after about a minute it switches off and the display starts blinking again.
Can anyone help me with this problem or give me an idea of how much I'm looking to pay to get the problem fixed?