Thanks for the suggestion to contact Used Subaru Parts @Mr B, they're checking if they have what I need.
I don't fancy the hassle of buying a non runner, stripping it, and selling what's left. It wouldn't fit in my stock trailer anyway!
I plan to keep the car for a long time so I don't mind spending some money on new parts if necessary; I'm glad to hear it's only £300-400 for the subframe - I was worried it would be a lot more. Sounds like I could get the whole lot changed for maybe £600-800.