and to remove the soap sludge (soap is no good at getting rid of soap), your hottest wash cycle with a cup of Washing Soda Crystals and no powder. If the water goes grey and foams up that is the old soap sludge dissolving. Do it again until it stays clear. You can put white cotton towels in, as they will not be harmed or faded by the very hot wash.
You may find it smells more after you have stated the service washes, this is because the old sludge has been disturbed.
To avoid this happening in the future once the machine is clean, make sure you use a bio or non bio powder and regularly wash at 60 degrees plus. Avoid liquids - liquids don't contain any form of bleaching agents and they're more difficult to rinse out, so they often leave a slime in the machine especially when used at low temperatures
you probably have enough towels and flannels to do hot hot hot cotton wash at least once a week.
It does tend to fade colours and patterns though, so white towels stand up better.
If your washer has a soap residue you may find you need no powder or softener with the towels, just Washing Soda Crystals, which remove soap residue and also grease from bath oil or skin lard. If the machine ever stops foaming, or there is make-up or black dirt, start using powder again. Towels tend to get impregnated with shampoo, soap and skin wash.