Next I checked the soap dispenser, and while that has obviously been
cleaned recently, and although there are one or two patches of algae,
they don't dislodge easily, and would not seem to account for the
amount of gunge, so I don't think they are the problem.

Any ideas?

Off to look at the shower now ...


Take a look at the door seal. Sort of pull the seal open and have a look
at the inner seal.
Chances are you will find horrible smelly gunge in there.

Sister in law recently bought a second hand washing machine, and everything
came out gungy and smelly, soap drawer was spotless, door seal was too,
drain to pump was fine, they couldn't figure out where the gunge was coming
from.

I had a look, took out the soap drawer and found the pipe leading down from
the soap drawer was thick with brown stinky crap, and the only way to clean
it out was to use a bottle brush fixed to a flexible rod... so we could get
right down to where the pipe exits into the drum,

one hot wash with a white towel later and it was much better, next wash was
perfect,

i can only assume it's because it was left standing for a while in the shop,
and the soap scum lining the pipe erupted into life growing the stinky slimy
****,