meg said:
... never at higher temp than 40 ....
50p says its the soap dispenser drawer. You can pull it out, I bet you will find it has congealed old soap and fab soft both in the compartments and the hole the drawer slides into. the fab soft has a couple of little pipes in it that you will see if you pull its lid open.
You can give them a good scrub with very hot water in the sink.
If you have black mildew on the bottom of the drawer or inside the hole, you can put some bleach-based kitchen cleaner on it. This will kill the mildew and once dead it will wash off. There might also be muck in the channel leading from the drawer to the tub, though I haven't found so, but your bleach spary and hot water should wash it away.
Some people say you should run an extra-hot cycle from time to time to wash away the soap, which also accumulates inside the drum if you mostly do cool washes or use a lot of powder. Or like me you can throw the powder through the porthole door onto your clothes before you start the wash (this is the easiest solution), or use a dosing ball.
BTW in my home I have a watersoftener and throw the powder in, and have a clean machine, my old mum uses the drawer and has no softener, and gets a lot of congealed soap and fabsoft.
Please send the 50p to my paypal account