I'm admitting defeat on this one, especially since I've already bought a
replacement second-hand microwave combination oven to replace it.

For reference, it's a Siemens HF83950NL:
https://www.siemens-home.bsh-group.com/uk/supportdetail/product/HF83950NL/02#/Tabs=section-spareparts/.

I bought a replacement part 15792 for it. This is what I think is a
thermal fuse. When I've looked up the part numbers marked on it:

Z430KK6E
E065FF1
UK T180

and pressed into the ceramic:

Z42/43
6/250
T250

that returns results that describe it as a thermal fuse.

Siemens calls it a "temperature limiter", which I guess might be close
enough. The old one has no continuity. Neither does the new oe I bought
and fitted - but rather thoughtlessly, I only checked that after fitting
it and turning on the machine, so for all I know it has immediately
blown, or maybe it's not actually a fuse and it's normal for it to be
open.

The odd thing is that when the machine is plugged in, one side of this
fuse is at 230V, and the other - following the cabling - seems to be
neutral (or at least, it the cabling disappears into what seems to be
the neutral/0V terminal of the "Capacitor-interference suppre." (part
no. 00065353) which is right next to the mains terminals.

So I am not entirely sure what sort of thing this fuse/temperature
limiter actually is, after all, and my oven is not fixed

I've spent several hours and ¤25 on this, which is disappointing, but I
haven't had an electric shock or microwaved myself, which makes up for
it, a bit.

Daniele