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In Feb 2023 we started to experience a foul, gassy, stink in our sunroom. I had to leave the windows open all summer. I got our local turbo-cleaners out and these discharged high pressure hoses into all the gullies, drains, manholes and sewers and found nothing. Environmental health were called and they said it was rats! After a few months the awful stench settled and was starting to disappear. For almost six months now I have had a musty, earthy, cabbage smell that changes depending on conditions - weather, heat, rain.
Yesterday, by pure chance I managed to locate the exact location of the smell, at its worst. It is coming from under a PVC cill board that is over the window opening in the sunroom. The outer walls are block and plaster, the inner walls are studded, insulated plasterboard skimmed. Any idea what this could be. I'm in the house 18+ years and this is all new.

Thanks

Could be anything, (Sorry it's not the answer you wanted) the only way to find out and solve it, would be to open the wall and have a look.

A few years ago, we owned a holiday cottage and after a couple of years our holiday home started to smell an awful smell, it turned out a creature such as a rat or mouse had died somewhere in a void underneath one of the bedrooms because it was a suspended floor. it took us a whole year for the smell to disappear as the creature slowly decomposed, it was awful but we couldn’t get to the area where this creature had died.
Another story was one of our relatives had a small gap where the sink drainage pipe went out of the building And somehow at some stage we presume it to be a mouse, got into the kitchen area and then died under one of the remote inaccessible areas of this particular kitchen and our relatives had to deal with the stink for quite some time.
You have two choices , either remove the windowsill and hope that you can see what is causing the smell or wait another six months during the winter whether the smell is likely to just disappear as the creature in your building decomposes.

Mike