how do I get rid of black gunk in washing machine?

[CandyKate]

A few months ago we were given a second hand Bosch washing machine. There was a lot of black gunk in the soap dispenser. I have used washing machine cleaners that you can get in e.g. Robert Dyus which include removing the soap dispenser and giving it all a good clean out, including the rubber seal. It seemed to work at the time but the gunk has come back, mainly in the pre-wash section that I rarely use. I've also noticed the washing having a vague mouldy smell all of a sudden.
What can I do to make it go away for good? Is the black gunk actually mould?

Boil wash with lots of bicarbonate of soda and vinegar should do the trick. Might take a couple of loads though.

The hottest wash, empty, with loads quite a bit of white vinegar seems to get rid of it. I have Bosch too - but it seems to come back after a while so i do the vinegar thing every 2 weeks. I take the drawer out and scrub it every now and again. The internal "hole" left by the drawer needs a bit of attention too - especially the "roof" bit

Thanks. Is it worth soaking the drawer in a bleach solution at all?

Remove the whole drawer and scrub it with an old tooth brush in the sink using an abrasive cleaner such as Cif.

Hottest empty wash possible with half a bag of soda crystals in the drum, you might have to do this a few times to shift existing gunk.

Only use washing powder, not liquid.