Ok, my first excuse for not replying sooner is that there was a glitch in the Community Site that meant I couldn't actually reply to my own post within this forum. I know, it sounds contrived but it's honestly what happened (remember the site was beta when I originally posted). Anyway, Colin kindly fixed it for me.
Nonetheless, the Comms and Parbin teams have been keeping the affected customers up-to-date with mail shots and website updates.
Understanding and configuring Plesk's overall system has proved to be complex - with various licensing requirements and integration points - therefore, it's taking longer than originally hoped. The last thing I want to do is migrate customers onto a new platform that we understand less than our current one, so the key here is to not rush into the migration process.
The good news is that we are nearing the end of build, and migration is now likely to start on Tuesday. We now believe we understand the migration process and better understand the Plesk system, so it's just a case of getting the servers fully commissioned before we start. We've already got the servers built for one data centre, we're just finishing off the other centre to ensure we have site resilience.
Thanks to Rupert and Mike for their hard work on this.
Dan