Remember that natural gas has about the same density as atmosphere, and therefore will travel around the property with ease, so the leak if small, will sometimes not be where you expect.
If the guy is Qualified and experienced, you have to take him on his word, after replacing the boiler gas valve, he will retest the entire gas supply system, and Will ensure that no drop (or leak) exisits
www.elanar.co.uk, August 2009