I found a replacement RHS metal strut online for £100 that seems legit and looks identical to my current one.
There are two spring clips on the existing strut that pop out with the help of a flathead screwdriver, then it just slides out.
De-wiring, or more specifically, getting behind the panelling is a bit trickier. I suspect the plastic panel just pulls off with a bit of force, but by that point I'd already taken off 5 different bits I probably didn't need to.
Once inside it's simply unplug, then plug and play with the new one.
If you find a Volvo dealer happy to do it for free, go for it, but these prices of £600+ are crazy if I can do it for £100 in under an hour.
Now time for a smug beer
