Thank you very much everyone for the replies! There are so many so I'm not going to quote them all, but that has given me a lot to investigate.

And yes, it smells like the cooker gas, so it's definitely that chemical that's added to gas (or something that smells similar - I have also noticed that garlic residue under my fingernails after cooking dinner smells like that!).

Dead animal: we did have mice in summer, so it could be that. It did occur to me but I didn't think it could last for four weeks and come and go, but if someone has experienced this with a dead rat then it's a possibility.

Washing machine: I've done a couple of "service washes" with mine recently using soda crystals and 95C water, hasn't made a difference but could be one of the neighbours.

A leak outside: another possibility - BT came and dug up our street a few weeks ago (1.5 weeks after smell had started) and seemed to be using water pumps. Our phone line had gone crackly so I think the heavy rain had flooded something underground. Surely if they had noticed gas they would have smelled it though, and we can't smell it outside (but then, it's not an enclosed space).

One of the neighbours' boilers is under the stairs, actually. The flue outside stinks of gas when it's on, but the gas guys tested the boiler along with everything else and said it was fine. I suppose it could be the flue smell, but I don't see why a) it would come back inside, and b) it would suddenly start now, after being fine for years.