Gas hob with no direct window to outside

Assuming the person was Gas Safe registered then they are probably correct. However you can always try ringing another person who is shown to be Gas Safe registered and ask them. They must be Gas Safe registered to work with gas as Gas Safe is the quango that tests them. You could always ring your Local Planning Authority too as they should have someone who can advise on these matters.

This is a very common problem and one I often have to deal with, ventilation is based on the volume of the room assuming you have a normal domestic cooker. If your lean to has opening windows then there is guidance from Gas Safe on whether the combined ventilation sufficient. You can also use an extractor fan, perhaps in a cooker hood to extract to outside. This depends on the size of the lean to but generally ducting through a ceiling and through an exterior wall should get you above the lean to. Gas Safe call this an internal kitchen and do provide guidance on meeting the ventilation requirements. The other option is to fit an electric cooker.