Thanks for the suggestions - looking into most of those!
@Mark - Crashplan sounds promising - further investigation required.
Balge - yes, my PC has eSATA, but the NAS doesn't. Therefore, anything has to run at network speed. I've been using Acronis True Image, but it's not designed to back up the NAS and I've had all sorts of issues with it. I've had hard drives fail in the past, so the RAID has to be mirrored!
JohnEBoy - Most of the data on the NAS is photos, so not much changes daily. Block level synchronise might be the answer. Is there any software you could recommend?
Balge - I've been bitten by hard drive failures in the past, so my current scheme is hopefully designed to avoid major loss of data. My PC and wifey's laptop both use the NAS to store all data - this is mirrored RAID, so hopefully one drive failing won't cause me to lose the lot. What I've been doing is backing up the data on the NAS to a 2tb harddrive that fits in a caddy on my PC - this hard drive then gets swapped once a month with another and then it's stored in a safe place.