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I think this problem is totally caused by Liquid detergents which don't have bleach as an ingredient. I no longer use them and the problem of smells and black mould has gone. I don't need to do a service wash and only very occasionally use 60 degree wash. I did initially run two 95 degree washes using detergent then distilled vinegar and scraped round all the door seals several times to get rid of all the brown gunk.
Hotpoint First Editon (WMA10) wash times - 115 mins for a cotton 40 degree wash 50 mins for acrylic 40 degree wash 45 mins for 35 degree wash (quickwash) 15 mins for a spin 30 mins for a rinse and spin