Thanks Bodger.
The reason I thought I'd need rewriteable disks is that I'd like to back up data weekly (that's the theory anyway!) and I'd have to use a single-use disk each time instead of updating a R/W disk. I suppose single-use disks could still be cheaper.
I've never used a CDR-W yet so know nothing about it. I understand it comes with Adaptec software. Is that product OK?