Thanks guys for all the comments.
I managed to dig out enough concrete for the gas guy to be able to cut the steel pipe with a reciprocating saw. He then put a blowtorch onto the screw joint at a T junction and much to my surprise he was able to twist out the pipe.
He then capped off the T joint and tested the system for leaks - there were none. He then put in a new section of brass pipe from the T joint, partially below the concrete. He wrapped that in tape.
So, for a job I thought could be a nightmare, it went quite smoothly.
Generally the pipe looked in reasonable shape, but as it moved from the T junction it became quite corroded. I think it's no surprise since the concrete wasn't as dense in one part of the floor and we've had a washing machine and dishwasher in that spot with various spills and thrills.
Anyway, one month from getting the gas cut off, we're warm again.
Thanks guys.