Just to report back on this.
Subaru dealer was not very helpful, since my car is a UK import, he wasn't able to access recall info and advised me to contact a dealer in the UK to deal with it.
I found steel wishbones on ebay, €50 used or €120 new, both are advertised as being for 02 Impreza but no distinction made between saloon and wagon. Because I wasn't sure if either would fit, I took a chance on the cheaper gamble. It arrived the day after ordering. All mounting points were in the correct position for my car but the front bar of the wishbone was thicker, appeared to be from a car with a lower subframe so it would have fouled the subframe on mine. I was able to modify the front of the wishbone to fit, its doing the job for now but I'd still like to replace with new or better condition items.
I couldn't believe how bad the old wishbone was corroded, something I hadn't spotted and I'm lucky the NCT tester did. If it had failed at any kind of speed I would have been in trouble.