Yes he is gas-safe registered. It was a small leak when he came and tested (I feintly smelt it but wasn't certain and he could not smell it when he came in at all), and after the tape/gunk was cleaned off the cap is when I say I could hear and feel it. Now there is new tape, you can't smell, hear or feel anything. I also have a smart meter which confirmed no gas is being used when I tested by putting the gas on and keep the boiler etc off, ie currently no leak. I have switched it off at the meter, and am keeping it that way until I can get someone else in who can can hopefully remove/replace the cap, and if not I'll be getting the whole gas pipework re-done.