Well, expense has started ... just had new drilled & grooved discs fitted all round. What a pain to source though, my friendly BMW Indy (who also races and BMW for a pastime) sourced a set that have been modified for the Alpina. They look great, although I need to treat them to a protective coating now to stop any rusting. Will take some pics and post up later today...
So, new discs (and pads) installed. To prevent the usual rust issues, the discs have now had etching applied - using a product called fast etch - that applies a rustproofing layer of zinc phosphate. The contact areas will still get surface rust (removed by braking) after each wash as the protective coat will be removed but all non contact areas should remain rust free.