GB Autoelecs on Longmoor Lane. Ask for Dave, tell him Pete from the Limos sent you. 0151 521 8766
Alternatively, I think it's reasonably easy to get a Ford Scorpio Alternator to fit the Perkins. I scrapped a Rangie with a Perkins 4203 last week, and that had a Scorpio alternator on.
Call me if you need the alternator dropping off @ GB, I'm going to be up in Rainford tomorrow morning / early afternoon, and it's no problem dropping it off on the way through.