Pressure relief valve. The plunger had jammed, and TBH looks a little scored. After cleaning it up, reassembling and priming I got oil pressure again, but a top end rattle
![Sad :-(](./images/smilies/icon_sad.gif)
Which was there on and off before - I don't yet know whether it's permanently rattly now. I was suspecting a sticky valve lifter, so will try a flush and some fresh oil (current oil is probably rather thin and petrol contaminated).
Didn't go much above idle with no oil pressure, so hoping not too much damage done as it's probably engine out just to drop the sump! Pondering that if it IS low oil pressure, rebuilding the pump with a modified case so it can use the later, taller rotors for more volume/pressure.
Filter is on the bulkhead with a remote filter head. REALLY hope it doesn't need a rebuild...