After installing the driver provided by Dell, I attempted to re-pair the keyboard using the standard 'Add a device' function, which could see the keyboard after I pressed its pairing button, but would spin indefinitely trying to connect to it. Search, Search, Search, finally found this post - deja vu sets in.
For some odd reason, Windows doesn't seem to want to automatically enable drivers for the keyboard. Right clicking the icon in the add devices wizard and selecting properties, then ticking the drivers box under services makes everything better.
Update: For some reason, after ticking this box once, Windows will automatically do its thing when you re-add keyboard after it has been removed for whatever reason. Re-adds will present an 8 or so digit PIN that you enter on the keyboard to confirm the pairing, which was was not asked for during the original pairing... I don't know if that is good/bad/indifferent, but it works.