Weird smell, what can it be?

Yep, if a clay pipe is running under the house and starts leaking, it could creep up into the concrete, I doubt your house will have a dpm under the screed or concrete unless it's been done fairly recently.

Actually, the house where I'm currently building a side extension, the owners years ago had their kitchen floor replaced under the insurance for exactly the above, it had been leaking years and slowly washed the soil away, causing the concrete floor to subside.

Lift the carpets (if you have them) and look for any darker patches, or look for any cupping or joints opening on wooden floors.

[Littlepeahead]

Hm, now you come to mention it we have a small porch on the front of the house and there is a crack where it joins the house in the plaster inside. Directly above the bay window the sill has shifted by about half an inch too. Could that suggest the front of the house has been moving if the ground underneath has been shifting?

Possibly, the ground moving can also cause the clay pipes to crack in the first place if enough.
Bays are notorious for shifting though, they never used to be built well enough, too much weight, not enough structure.

[Bibio]

do you have Norwegians living next door...

[Littlepeahead]

No, but they have put a bloody great gnome out the front which I'm not happy about.

[Bibio]

is it a carnivorous gnome...

[andrewsmith]

They're Norwegians and have opened Surströmming

[Bibio]

before you go ripping carpets up and getting jack hammers out.

is the room an extension on the house?

[Littlepeahead]

No the living room is part of the original house, Bodger Pete the previous owner, built an extension but that is our garage with a bedroom above. The garage smells of garagey stuff, motorbikes and bacon if I've had the extractor fan on, not zombie breath. I should point out for anyone thinking of visiting this isn't an overwhelming stench like open graves, but it is annoying!

I doubt it is anything dead, unless the cats are regularly bringing in dead things and putting them in the same place each time, but you seem to have ruled that out. If it was something dead it would have either rotted away or totally dried out. I know nothing about drains, only dead animals because we were always being presented with them by the cats and dogs on the farm when I grew up.