I'd recommend just getting a local heating engineer to come and take a look at it really, after all they'll be able to diagnose the fault and fix it pretty fast usually.

The last time I had a problem with my boiler, I was going to use BG but with the call out fee's they mentioned I thought "no chance". I was wondering what to do, then I heard a radio advert for some firm called STL Heating - "well, what the heck" I thought, these guys are local - why not ?

So I gave them a call and thankfully the call out fee was a lot less so I've been using them ever since for my boiler service.