Gas leak all over south Wales any ideas what it is?

The only time when I was in a building and smelled gas and went outside and it still smelled of gas it eventually turned out that the reason was not gas but the additive they put into gas to make it smell. Natural gas does not have a smell, usually mercaptan is added to make it smell. The mercaptan can leak from a place producing it and make wide areas smell.

Here a report where it happened years ago: http://www.bbc.com/news/world-europe-21147361

If wide areas smell of gas, it is most likely a mercaptan spill somewhere. Surprising little of it can cause a big stink - that is one reason why it was chosen to add to natural gas. Mercaptan is not really dangerous, though the smell can be nauseating. Natural gas is lighter than air, so outside it should rise, I am not 100% sure but since mercaptan is a larger molecule it is probably heavier than air, so the smell would spread.