I will investigate more and get back, but I have a feeling that when the kitchen extension was built long before I bought it, they may have routed the gas pipe outside along the side of the extension. There is around a 20cm gap between my extension and the neighbours extension and has been piled with shingle up to the damp course. My feeling is that I may find the pipe along there and it comes round the corner under the ground and in through the kitchen wall as in the photo. What I’ve just realised is a few years ago there was a hole coming out the wall which was probably for ventilation as there are gas pipes running behind my kitchen units. I filled it up not knowing what it was for but think now it should have been left for ventilation in case of a leak. Have included a photo of the gap between the 2 extensions which shows a weird gap at the end.