It soon became clear that the Preview update package supports only a small subset of languages and while US English is of course supported the update would refuse to install on a device running UK English.
I first followed these instructions to install the update package:-
This worked and enabled the Store item to upgrade but that itself would not work because of the language issue. I then found this answer on the forums with a solution. It requires editing the registry so use it at your own peril:-
After this (reboot required) I was able to start the update process. It requires a 2GB+ download so took a while followed by quite a lengthy install but has now completed and I’m running 8.1. After the update I was able to set the keyboard language back to UK English, but you can’t set it as your primary display language because there is no UK English language pack in the preview. This isn’t a big problem as I’m already used to working regularly with US spellings.