Thanks everyone!
My machine doesn't have a 'clean machine' cycle but the hottest wash is 95 degrees so i will run an empty wash on that setting. I will try Bicarb and see if that works, then bleach if i have to!
I always leave the door open after a wash and regularly clean the seal but it has never been this bad and full of slime before, and it has never stank! I only ever wash in cold as about a year ago DH thought he had put on weight as all his tops were getting tight, turns out they were just shrinking in the wash!
Will let you know how i go!