Deleting the old restore points seems to have fixed the problem. There were a few others around which appeared to have invalid files. I simply deleted them to clean everything up. Thanks for you help.
I'm sure I am seeing the same issue actually. As I suspected there to be a discrepancy I let my restore points build up over the last week, used JKDefrag to move system volume information folder to some free space to see it clearly. Deleted all available restore points with CCleaner and the block of files shrunk by about 30%. Then ran disk cleanup's delete old restore points and the block of files shrunk to about a tenth of the size.
WinXP Home, CC2.21 (I know it's an old CC version but as far as I'm aware the restore point code hasn't changed in subsequent releases so though it was worth posting)
Have just done the above again to to see if I could repeat it and sure enough running disk cleanup's "Remove old restore points" after deleting all old RPs with CCleaner removes more data than CC did.
Surely a bug or CCleaner missing something it should be deleting.