Identify this strange smell?

[TurquoiseSeasAndSilverSand]

Our downstairs bathroom has started to smell strangely chemically. It smells very similiar to heating oil and smells the worst first thing in the morning.
We do have oil heating, but the boiler is in an outhouse and the tank is outside, neither smells.
I've taken the panel off the electric shower and the wiring is all fine, no burning, and the shower switch isn't in the same room.
I've checked under the bath, emptied the cupboard and the floor is vinyl, so no spillages or anything. Bin is clean and empty.
The u-bends are clean.
We're at our witsend with it, it's getting worse. I'm posting here for traffic, in the hope that someone has had a similiar experience and has a solution.

Have you checked the heating vent? Could fumes be blowing in somehow? Incidentally, do you have a carbon monoxide monitor?

We do have carbon monoxide detectors as we have solid fuel fires. The boiler vents through the roof of a separate building, which isn't near the bathroom and it doesn't smell outside.

Have you had your chimneys swept? Also, if cold air is pushing the air in the chimney down and out into the room - through the stove vents - this can smell. I sometimes get up to this on a morning. It goes away when I light the fire. It has a kind of fishy tarry smell.

Scent plugin that's gone dry?

I get a Jeyes Fluid smell in my downstairs bathroom occasionally, which is odd because I don't use the stuff and I live in a detached house. I suspect a neighbour uses it and somehow the smell gets into mine through the drains - which should be impossible and there's never a 'drain' smell. I suspect it's something to do with strong winds through the soil-stack vent, because sometimes the water in the toilet moves a bit in very windy weather.

Or you don't live next to someone with a hard-to-start diesel do you? Unburnt diesel from a cold start in the morning smells and it would get worse with the cold weather.

Thank you for the replies.

It's not stale chimney smell and I don't use any plug ins or room scents of any kind.
I am wondering about Jeyes fluid and drains, we don't use it, but it is a similiar smell combined with oil. We do get drain smells occasionally, as water gets pulled out of the u bends. I am wondering if one of our neighbour's oil tanks is leaking and getting into the drains. Although the washer is connected to the bathroom sink and it doesn't smell.

I think my next step will be to put the plugs in over night and block the overflow holes and see if we can still smell it.

And no dodgy diesels on our row, ours is the roughest and it starts fine.