Hi, do you happen to know where the smaller branch from the supply to the cooker goes? The original (can tell it's original as it was behind 1940s plasterboard) pipe goes from the meter up inside the hall/kitchen wall, comes out into the kitchen just below ceiling level (to avoid a steel beam in the top of the wall) then goes up into the ceiling/floor void, across the top of the kitchen and down inside the boxing in of the soil pipe to the cooker.
Where the pipe comes out in the kitchen, there is a smaller branch that goes straight back into the wall into the 'channel' of the I-section steel. Can't be feeding anything anymore as the only other gas appliance is the boiler which has its own supply.
Funny you say it's difficult to work on a BISF house as I've found it fairly easy as all the walls are stud walls or dry lined, so it's easier to put pipes and wiring in them. I've already rerouted the DHW and Ch, put in the condensate etc in the walls/loft etc.
Ed