No...not a wind-up. Renovating this old cottage. Took it back to the original shell. Smelt pretty pongy at times. Thought it might have been the drains but they check out OK.  Thought it might be a dead rat but since all the rooms were back to the bare walls and the ceilings were open - both between ground floor and first floor and also open to the roof space totally, couldn't see anything at all.  Then it went away.

Now after dry-lining and making good, it's back but not all the time. Any suggestions?

Here are the more wackier suggestions I've already come up with or discounted but really scratching my head here.

1) Don't think it's SWMBO farting

2) No dog so its' not that

3) Poltergeist.  Suggested it to the missus who freaked out. Mind you, the next day when she wasn't there I smelt it again and so told the poltergeist to f*** Off and no smell today and SWMBO is back on site....see suggestion (1)

Can old brickwork/chimneys smell like brussel sprouts?

Thanks for the suggestion but already checked that one  

Could you have cracked an old pipe or something whilst you have been doing some work?

Wolf - good idea - need to check - which addresses Jerry's point.

Sandy - no pipes to crack. Actually, that's not quite true come to think of it. There is something underneath what was the conservatory but now the new orangerie. Maybe an old well but capped and sealed and never touched. Then cemented over with self-levelling compound, cement and tiles.  One of the rain water pipes does empty into a clay pipe that heads off in the direction of said well. But it really is capped and sealed good and there is no other entry - we excavated the inside of 2/3 of the ground floor prior to laying the dpc and concrete floor.  There was one other pipe entry from inside the orangerie and I sealed that off and it's behind a stud dwarf partition (that is still open at the top so I can sniff for smells - and did check it for any smells when the brussels came on-stream but it was clean as a whistle.

Have you tried a different deodarant? Brussels for men was never a popular seller.

Dead bird up the chimney ?

What's the soil smell like? I grew up in Surrey and if you dug a few feet down the soil itself had quite a strong smell.

Interesting idea.  The rain was a red-herring on my part as we've not had any rain for the last 7-10 days or so and it still is there.

Investigation of the soilpipes/sewage system update...

The smell came back this morning and as things thawed out a bit I decided to investigate the above a bit further.

The tank is a septic tank. I checked the level and it's about 5 ft below ground level. Opening the cover to it didn't give off the same smell (or anything like).

Working back, I opened the inspection cover to the first downpipe. This is a new downpipe and takes waste water from the ensuite including shower/basin/wc. There is a short stub pipe with an air admittance valve boxed up behind the stud partition. I've got a mains socket in this partition by the pipe but removing the socket and sniffing has never shown the smell. Also the downpipe was exposed on the other side and again no smell apparent even when the smell was elsewhere.  So I kind of ruled this pipe out.  Opening the inspection cover didn't give off any smells like brussel sprouts..in fact quite sweet.

I did notice that the flow was lazy and rodded the pipe between this inspection pit and the septic tank and cleared out some old rags.  Then went in for lunch.

While lunch was cooking the smell went away. Now that's either because it was masked by my lunch or it's gone away.

So I've left the two inspection pits open and will wait and see if the smell returns.

Anyone got Ghostbuster's number?

Sorry, Londonman, when you're in the middle of this sort of thing your sense of humour tends to get a bit strained. I know you've had your nose up there more than once, but I still keep coming back to the chimney because of your description of the smell, which need not necessarily come from the actual chimney place but from the brickwork, or rather the mortar which, in an old house, will be lime mortar and that does react with the sulpherous deposits left from burning coal producing calcium sulphate which actually causes the mortar to weaken and expand. I'd be inclined to cut a section back to the brickwork and inspect/smell the mortar joints. If it's not that then I await your discovery of the actual cause with real interest because every other contributor seems to have covered every other eventuality. Good luck.