In reply to JimR:
I didn't get a reminder for my road tax this year (£0, due to emissions, but still need to "renew"), but then I've noticed a couple of other letters not arriving, so I deduce it's Royal Mail not dvla.
However, I believe no reminders are sent when a driving licence is due to expire (where there's a 10-year expiry), which would be quite easy for dvla to do a reminder letter, which is a bit naughty as the penalty to the licence holder is quite high, as someone else pointed out.