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Folks, I finally found out the source of the smell. Not burnt out plugs, dead/frisky mice or fish in the works. It was milk (yes, I have been crying over spilt milk!). I remembered I had knocked over a jug of milk in the fridge last week and it must have gone down the drain hole at the back. It has been busily turning itself into stinky cheese ever since (and getting riper and riper by the day!) Woofgang, you were on the right track. I had thought about Milton myself and put some down the drain this morning. Smell not totally gone but cooked Milton definitely smells better than cooked Stilton! Thanks so much.