Who is my favourite doctor.

Try putting a cup of white vinegar in your drum and run a 90°C wash.

You should manually clean as much out as possible first with a damp cloth, then run the cleaner through.
As an aside, avoid using liquid detergent, these don't tend to have Oxygen based bleaching agents (bleach) which when combined with our habit of washing at low temperatures results in the slime. The powders help stop the slime in the first place, especially if you run an empty boil wash once a month.
Also make sure you're using the correct dose as too much is just as bad as too little.

Calgon is to get rid of limescale, not the bacteria and sludge.

I use something called "Professional Limescale and Detergent Remover" (sorry, that's all it says on the box)

Box of 12 sachets, use 1 a month. Costs me £15 a box from my local Appliance Shop. The only other thing I can see on the box is that it's approved by Hotpoint/Indesit and is manufactured by Docchem Srl. Also says distributed by Indesit Company S.p.a

Put 1 sachet it the detergent drawer, set it to 60 degrees without a prewash.

Tried Dr Beckmans and had no joy but this stuff is amazing! Cleaned mould off the seal and the machine smells spotlessly clean.

And this is all using Ariel Liquitabs so you don't have to go back to powder if you don't want to. Used it on an old Indesit washer, now using it on an LG Direct drive washer dryer with no problems.