Just following up on this for anyone who comes across this thread...
Turned out the alternator fitted was some generic aftermarket one and I couldn't get a replacement voltage regulator for it. I picked up a genuine Volvo alternator with regulator second hand from eBay, fitted it with the aid of a crowbar (let's just say the aftermarket one didn't fit properly so hadn't done the mountings any favours) and job done, problem gone.
Thanks for everyone's input on this thread!