Apologies for multi-posting, but I realised after posting this to uk.rec.cars.misc that I'd be likely also to get some knowledgeable answers in uk.d-i-y.
Recently my car was stuck in a ditch over a weekend and mud got into the front passenger footwell. Since then, I have cleaned the fitted carpet using spray-on "Carplan" car upholstery cleaner and also "Rug Doctor" carpet odour remover, both times scrubbing it into the carpet with a brush. I have unclipped the casing at the side, bent the carpet up, and cleaned and dried under it with rag and tissue paper, and I have left a fan heater on in the car for hours to ensure the footwell is completely dry, including both the carpet itself and under it. But there's still a pong.
Advice would be gratefully received.
Thanks in advance!