Ok thanks Dan.I understand what you're saying mate.
If it was something more to do with the water side there might be a hope in helping but with it literally being the gas side of it, nobody is allowed to.
Get 3 quotes. Don't go for the cheapest.
I didn't originally realise that you're not allowed to share information on potential causes of gas faults. Is this a Gas Safe Register restriction (or other authoritative body), or is this a forum restriction? I personally feel as if it's a bit daft, as the people who are stupid enough to play around with gas when they don't fully understand what they're doing will probably end up blowing themselves up whilst tinkering around regardless of what information is out here on the internet