It was definitely after 1966, but before 1969, as we still had town-gas at our maisonette in Gleadless Valley when we moved in there in 66, just before my sister was born. (and it was installed before I started school in early 1969)
I remember it happening, and I remember the really old-fashioned gas-stove my mum and dad had, at the time. it was old, and grey-and-white enamel. The engineer had to alter the burners on it as natural gas had a different pressure to town gas.
I remember my mum and dad poking fun about the new North Sea gas, teasing us, asking "Doesn't your food taste of fish, then? It's been cooked on North Sea gas!"
Apparently, you couldn't "gas yourself" with Natural gas, like you could with town-gas. Natural gas also had no odour, so an odour was added to it, to help the customer smell if there was a leak.
"I can't afford to live" - Stephanie Bottrill, May 2013, victim of the Benefits "Reforms" R-I-P