Steve,
This is the usual grey muck I take it? This is normally caused by several years of doing 30/40c washes, combined with failure to clear the filter. The problem is exacerbated in hard water areas. I live in a soft water area, and a cheap fix is to clear the filter (under the kick panel) and boil out with a cupfull of bleach. Hard water people seem to prefer soda crystals, but the basic principle is the same in either case – make sure the filter is clear, and boil out.
In some cases, an engineer can pull the machine to pieces, high pressure water and a scrubbing brush will scour the clag off, but allow for AT LEAST 2 hours work.
The Ariston A1234 has a sealed tank assembly (great), so bleach/soda is the only option. If it kills the odour a little, do it again!
Regular hot washes from this point on will help.
Regards,
Penguin45.