'Washing Machine Smells' are caused by smelly bacteria that breed on the residues of washing powders and washing liquids that adhere to the sides of pipe work etc. And, because many of the washing powders and liquids are designed to work at lower temperatures, the smelly bacteria are not killed off and the powders and liquids are not dissolved away quite like they used to be. The build-up of limescale in hard water areas will also increase the risk of washing powders and liquids adhereing to the sides of pipework.
To resolve the problem, use 'Smelleeze' from eco-Natura. 'Smelleeze' contains friendly bacteria that out compete the smelly bacteria and quickly gets rid of the smells; they will even eat up the residue in the pipe work. It may take two or three doses once a week on a rinse cycle, but it will last for ages; and it will stop your drains smelling too.
Also, if you find you are having black mould in and around the powder/liquid dispenser drawer of your washing machine, then you can clean with the same stuff and it will keep the mould at bay for ages.