In the end, Apple just sent me a replacement for the original. I don't think they're still available, but the 'holding fee' in case I don't send back the faulty one was £135, so Apple have a stock and would still supply them?
There is a list of compatible cards for specific machines on the Apple site. I did try a stock PC one, but they don't work unless you hack into them, which isn't straight forward. Good luck.