Well after a bit of head scratching all went well thanks to great support here.
Here is the procedure I used.
1 Created a new Debian Wheezy VM on BigV using the web panel (Sadly missing a re-image option at present. Using the command line was simple though) with 25GB main disk and 50gb of archive space.
2 Installed Symbiosis RC3 Thanks to these instructions from Joshr
3 Edited my firewall and php.ini file to match my old VM then re-booted the machine to check all was running correctly.
4 In the /srv folder of my old VM one by one I .tar each domain that needed moving. One is accessed quite often so I needed to make the folder read only first to prevent writes. I use WinSCP for this bit as it saves a lot of typing, just right click and there is an option under "custom commands".
5 On the new BigV VM using the command line (easy with the vnc button on the bytemark control panel) used SCP to copy the .tar files over from the ol VM to the new /srv folder. Once again, using WinSCP I untar the files where they are now in the correct place with all the right permissions in tact. Only 1 folder to re-enable writes on, also adjusted in WinSCP. You must remember though to delete your /config/dns folders so the symbiosis script can generate new ones base on the new server.
6 Thanks to the tip from Tony Rogers I .tar all my databases from the /var/lib/MySQL folder and again SCP them to the new server and untar them in the correct place. A quick check in phpmyadmin showed they are all present and correct.
7 Using the test facility of Symbiosis I tested each site in its new location to make sure they were all running an connecting to the databases as required. The docs show the old naming for the link (http://symbiosis.bytemark.co.uk/docs/ch-website-setup.html#testinga_new_domain) but with BigV it is slightly different. For example if I had the domain my-brilliant-site.com on a VM called mynewvm made by my username, phill104 I would need to use the following link http://my-brilliant-site.com.testing.mynewvm.default.phill104.uk0.bigv.io/ , maybe itwould be good to adjust the Symbiosis docs to reflect this
8 Next I had to contact Bytemark to get the DNS Authorisations changes. Thanks to Nathan Lasseter he explained that I needed to supply the DNC account names from the old and new VMs. In Symbiosis these are found in the /root/BytemarkDNS/upload file on a line that starts RSYNC_USERNAME= . I passed this on and a few minutes later all was working on the new BigV VM.
9 The final task was to get the backups in the correct place. For the moment I am going the simple route of mounting /var/backups on the archive disk but later on I may expand this as per the docs mentioned earlier in this thread and use this backup space for any other VMs I may create. In the aforementioned docs it says you can simply mount /var/backups but it doesn't outline how, it would be great if that could be documented too.
I've put this here partly to help others in the future, partly to remind me what to do next time around and partly to say thanks to all those that have helped me get this right.