Just thought I would update you guys in case someone else finds this useful in the future. So I couldn't find any replacement black plastic high-spec tilt Volvo siren YWK500050 or YWK 500070. I ended up buying a metal high-spec tilt FoMoCo siren from a Range Rover 6G9N-19G229-DG. Used examples can be found as cheap as £15 on eBay, although it may be worth swapping out the battery for a new one (and perhaps trying to mount the battery externally so it can be easily replaced and doesn't leak acid and fry the circuit board in few years time). I had to modify the old bracket slightly to make the new-style siren fit but it was very straightforward. Alarm system service message has gone and I tested the siren is working by waving my hand inside the cabin with the car locked and window open.

 

My alarm error message finally went away...
Don't ask me how I did it because I have no idea

 

So the YWK500050 version is the high-spec one with the inclination?

 

Weird thing: I lifted my car today when reworking the side markers (it was locked) and the alarm kicked in... even though I have the alarm error message on the dash