Ok this incident was not in the UK but in Hungary where we've just returned from.
Had to change accommodation for a number of reasons, one of the reasons being that the apartment we'd booked was FILTHY (think grime on the work tiles, huge cobwebs on the ceiling and a mattress stained with blood); however the main reason was that the apartment smelled very strongly of gas.
The smell was coming from the gas meter which was inside a cupboard in the kitchen. We noticed it as soon as we arrived.
The landlord was called and was totally NOT bothered. He (grudgingly) agreed to come round that night and check.
We opened all the windows and waited.
When he arrived at 11pm, he stuck his head inside the cupboard, muttered something in Hungarian, and said "is fine" to us. I replied that it's not fine, it stinks of gas, and a meter should not smell like that.
He sulkily said"ok I will return tomorrow and sort it out" and disappeared out the front door and ran away.
We were shocked and disappointed. We called him again but he didn't answer.
The smell was very strong and I was really worried. DH booked us into a hotel down the road.
The next morning we called him at 9am and he didn't answer. We spent the morning sitting in the apartment smelling the gas with the windows open and not knowing the best course of action, until he finally turned up at 1pm with another chap who turned out to be the caretaker of the building.
He too stuck his head in the cupboard, said "is fine" and then proceeded to flick a lighter and then held it up next to the pipes?!
DH asked him what he thought he was doing, and the guy replied "Look no gas!"
I said to the landlord, "You do realise that if there had been a gas leak we'd have just been caught in that explosion??"
He smirked and said "yes but there wasn't."

They eventually left after repeatedly telling us "no gas."
Ok so the "lighter test" proved there was no gas leak, however after they left the smell became even stronger and we could smell it everywhere. Bathroom,bedrooms, kitchen. It was very very strong next to the meter.
I couldn't relax and after we discovered how filthy the apartment was we packed and left after 3 days.
The landlord has been totally ignoring our calls.
On the day we packed and left there was a strong smell of burning in the apartment as well!!