"It's a (partial) blockage in the pipe between the soap drawer and
drum."

that's what I thought but I can't feel one, and can feel right to
where it joins the drum. The pipe is very flexible, I can make it
meet in the middle. I've never had a washing machine to bits and can
imagine all sorts of bizarre syphonic madness that would confuse
common sense.

I can't see how to get at the pipe further - would I have to take the
door off and seals, or take the soap wotsit right out?

I'm sure there's a good reason for it but it would be nice if the
sides came off!