OK, time to hold my hand up and admit to being an idiot.
I went to remove sda from the caddy to discover that the disk wasn't properly located. Plug in it and voila, I can see it. It's now back in the host server, where following Steve's advice I had confirmed I could see it on its own, so it now has my new drive along side it. However, when I try to rebuild the array I get the same errors as above.
At the moment this is feeling like a controller issue? (Or maybe PSU?)
Mark -- Mark Rogers // More Solutions Ltd (Peterborough Office) // 0844 251 1450 Registered in England (0456 0902) @ 13 Clarke Rd, Milton Keynes, MK1 1LG