I had this exact problem with mine and it drove me mad. I would walk in my house and smell it right away. Leaving the door closed made the smell worse and leaving it open to dry made the house smell. There is one product you can buy
http://www.ciao.co.uk/Dr_Beckmann_Service_it_W ashing_Machine_Cleaner__6963840 I have only seen this in Sainsburys and some Tescos
But I must warn you if your machine is really bad this will only make it smell nice for a few days. I had to buy a new machine and now use this once a month and keep the door open all the time and wipe round the seal after every wash, so far my machine is perfect. Good luck