While putting the world to rights over a pint of Theakston's Best last night, the subject got round to the above.
I was a youngster at school when the gas supply changed from "Town gas" (as manufactured from coal) to "Natural" (or "North Sea") gas; must ahave been the very late '60s? I can remember that all gas appliances had to be modified to suit the new gas, and we got new Bunsen burners; I can recall my science teacher telling us that otherwise the flame would have reached the ceiling.
ISTR there was like a 'D-day' when the conversion was done, but I can't remember or imagine how this was done in practice? Does anybody older than me recall? How big an area would they have had to do at once? How long did it take? and how did they get the whole system safely flushed out of the old stuff before everyone could switch on with the new?
Etc Etc...!