Running ‘clean-up’ of nonstop backup does not release any disk space.

Having been using nonstop backup for some time very happily. The target drive was becoming quite full. I ran the clean-up option to delete all but one month. This appeared to run successfully but after it had completed there was no more free disk space on the target drive than before I ran the clean-up. I tried re-running clean-up but according to the utility the previous backups, which I had meant to delete, no longer existed. I tried a reboot and a drive scan but still no more space had been released on the target drive. Am I missing something here – is there another step I need to run, in order to release the space previously taken up by the back-ups or is there something wrong with the product? The only way I have found to release the space is to delete the nonstop backup from the target drive (after running a full disk backup to a different drive) and start it again but obviously this is not desirable.

The product is up-to-date. I run the product on a stand-alone physical computer backing up the internal hard drive to an external removable drive. I am running under Windows 7 Home Premium SP1 with all patches installed. The internal drive is 700Gb split into three partitions and the external drive has a 1Tb capacity.