Mildew in Washing Machine

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I have some lovely lovely tenants who have just written to me to tell me the washing machine is leaving black bits on their washing. They think it is down to mildew in the drum. When I last saw the machine a year ago there was some mildew inside the rubber seal and I took to it with a mildew remover spray, bleach then vinegar. None of this shifted it but it did not look like it was going to 'flake off'
Has anyone had this problem and can mildew build up inside a drum or just in the seal.
I am desperate to resolve this issue through cleaning as it seems a bit over the top to buy a new washing machine whilst it is still otherwise in perfect working order
Many thanks for any advice

hi- mildew build-up in washing machines is usually caused by constantly running machine on low temp washes ie: 30 / 40 degrees and using washing liquid makes it more likely - you can try to clean it by running on hottest programe with proper washing powder & a little bleach then run programme again without any detergent to rinse it all away - you also need to pour some hot water down the soap tray because the mould can build up in the pipe that leads down to the drum - good luck with it & also letting them know they are the cause of the problem

Thanks so much DDD - your reply suddenly triggered a memory of pre children when I used liquid cleaner myself - mould used to grow on the underside of the inside of the cavitiy in which the dispenser drawer sat - I bet this is where the black stuff is coming from (only use fairy non bio powder in economy packs now I am mother of two given the number of washes I do). Willtry all your tips but also suggest that they have caused the issue (as I do sort of feel they might be angling for a new machine and it is a really good and not that old Bosch)