You were robbed at £70 unless they are the 312mm mk4 caliper carrier brackets.
Anyway buy some large bottles of Tescos or similar own brand vinegar (very cheap) and get a suitable size container. Cover the brackets in the vinegar and leave to soak for a few days. All the rust and sh!t should just fall off with a bit of brushing. If not leave to soak for another few days.
Good luck with your sale.
Paul