I use A & P Contractors Ltd 020 8366 2236 or 020 8366 2714
This is the outfit who used to be on Green Lanes as C G Gas Ltd (I think).
I second that. Have used them for twenty years. Reliable.
Also look in "Recommended Trades & Services"
Has your issue started today? we have a low gas pressure error with our boiler and wondering if it's an issue in the area?