website 'forgets' tariff when moved from 'default'

[mjeveritt]

If you change (eg. agile) tariff from eg. April 24 to Oct 23 (which I appear to be using) .. a refresh of the website resets the tariff details. I'm pretty sure it should be fairly trivial to save this value in a cookie to be persistent across sessions.
Whilst there are annoyances with storing data in this way, there are a lot of pre-baked solutions to displaying a warning message at this time!
(this will also be necessary as/when you locally cache customer API key info in due course)