Is your gas safe engineer fully qualified? Any engineer working on your appliance must be a fully qualified and registered gas safe engineer, you can check this by asking to see their gas safe ID card.
If you feel that the appliance is not safe to use then isolate the power supply and turn off the gas to your property at your gas meter.
If you smell gas, isolate your gas supply at your gas meter, open all your windows and doors and do not turn on any light switches or plug sockets.
I was very pleased with Sean and his colleague who came and repaired my boiler. Highly recommend this company and shall be asking them in the future for any plumbing and boiler job!