Thanks all for further comments.
The driver is ‘unknown’ as, although it was one of two, we genuinely don’t know which one it was – hence ‘unknown’. The notice of intending prosecution arrived 19 days after the event (although it was dated only 9 days after). We do not know who was driving at that time 19 days previously – as mentioned, we do a lot of local journeys, the i3 being the car of first choice. If you share a car with a partner and use it similarly and you can remember who was driving nearly 3 weeks earlier, you have a better memory than me.
All that said, one of us will own up to it. Neither of us has any points and neither of us have the time or inclination to fabricate some ludicrous yarn about lending the car to a fictitious person from a fictitious address (a ‘bloke down the pub said…’ story if ever I heard one!). More pertinently, neither of us wants comeback from having our mobile phone records location-checked - we don’t seem to have Google-maps tracking enabled (a reference to someone’s earlier post), but presumably this data is available to the police from phone providers (? - no idea if this is true), so one of us will bite the bullet and avoid unwittingly perjuring ourselves.