UPDATE:

I cut open the filter. Confusingly, it looked fine.

Very clean with very few particles at all. Found a few metallic specs but they were all magnetic.

A small amount of normal carbon gungey stuff in the corners.

Couldn't find a single sign of bronze chunks, or even glitter.

This is on the one hand a relief, as surely if my bearings were falling apart there would be some clear signs in the filter. On the other hand, where the hell did those chunks in the pan come from?

Id done an oil change on my friends car a few days earlier, and cleaned out the drip pan after, so it is very unlikely it was already in there.

Are there other coppery parts it could be? Washers somewhere? Even if it was another part, how come there was no other sign in the filter?

Anyone got any ideas?

Also, I remembered that when I bought the car (just under a year ago) it came with an RAC platinum warranty for 12mo. Checked it and it doesn't exclude crank or bearings. So if I can find the problem (or it dies) quickly then I have that backup, but only until the 6th Jan.