The error I'm getting is a read error on the verification pass. The verification pass is fine to about 85% then it starts having real problems. However, it's done that on all 4 disks, which is why I'm so puzzled by it. It could be the media, but it seems unlikely given that I've tried 4 disks.

I just burned the ISO, no unpacking, etc, just a clean burn.

I haven't actually checked the size of the disks, and I don't have another one to hand. However, the burn pass went fine, I'm assuming if there wasn't space on the disk it would have failed during the burn pass. The same option exists in Nero Express, accessed by clicking More > Disk Info. When I get (steal from my neighbour!) another disk I'll double check that just in case.

It's a mystery... Callum.