My valiant turbomax plus boiler is giving no heat help it's snowing out!?

I have hot water but no heat if I put it in sf.s mode I get heat for about 15 mins then it goes off again. The status display reads s.34 which means no call for heat from low voltage controller (not sure what that means though) the controls are turned on full for heat and water but ignition doesn't seem to be automatically firing up except for when I turn a hot tap on then I can hear it kick in and I get hot water. I dont think anything broke cos it was working fine till my husband messed about with the pressure realease valve ( lord knows why he felt the need to mess about with something that wasn't broke) the pressure at present is sitting in the green bit between one and two bars. Please help it's freezing cold :o(