Thanks guys.
Knew there was a simple solution to this!
Edit: Just had a check and our meter is one of those where the numbers rotate (like an old digital clock) rather than one with a rotating disc.
Do you think a reflective optical sensor (or rather half a dozen of them) would be sufficient to pick up that movement? I'm guessing it probably would and I could simply count which of the sensors is switching; units, tens, hundreds etc