Firstly, thanks to everyone who offered suggestions.
I eventually came up with a good way to solve this one and thought I'd share.
Just to recap, the disk I wanted to shrink originally contained three partions,one system, one boot, and one data, I have cleared (and deleted) the data partition and wanted to resize the disk to reclaim the space used by the data partition.
The host didn't have enough free disk space to create a new disk and copy the data to it, but another host did, so I used the standalone converter to copy the machine to the other host, but the trick was to start the machine and copy it as if it was a physical machine rathen then copying it as a VM.
That enabled converter to copy the mounted volumes rather than the whole disk and effectively recover the unused space, once the copy was complete, I could then delete the old VM and then transfer it back.
I did this between two hosts, but there's no reason why it couldn't be done just as easily copying back to the same host using a different name.
Cheers everyone,
P