Hi all
Thanks for your suggestions so far.
In terms of the hob, on this set up there's no way to turn the switch/ dial without pressing it down. My process has been to switch on the lighter and hold it by the flame ring, press down the dial and turn it to flame and then wait for a few seconds but nothing happens or, rarely, it feels like it might be starting to ignite but stops without me turning the switch off.
In terms of the gas I can't hear the hiss but can still smell it a bit. I knew about the gas bottles possibly running out but was thinking it would be too much of a coincidence for them to run out straight after the survey, maybe not though! I've got two gas bottles and from weight they seem roughly the same which could either mean they're both empty or both relatively full. The hob is my only gas appliance.