In May 2016 there were 38,537 registered Electric Highway users. If you gave everyone a 5 digit number with a single check digit then a randomly guess low number needs 10 attempts to find the check digit. If you make the numbers completely random and distributed evenly across the 6 digits you still have a 1 in 25 chance of guessing a number to give yourself free charging which won't take long even with lockouts on pumps after some number of failed guesses as once you've guess it you can re-use the number you guessed or post it online.
Introducing a letter just makes for something even worse to remember.
If you're going to have an identifying number then stick it in an RFID card or (better) in a mobile phone app. Then it can be as long and random as you want.
A randomly generated - perm any 6 digit from 10 - number would be almost impossible to recreate randomly by guesswork within a few tries. I realise that if there were 38,000 such random selections out there then happening to fall on one of those by chance is increased, but even then it would be like winning a lottery worth £6.
Introduce two 'alpha' selections and you would have to be a particularly diligent thief to input twenty thousand attempts before landing on one that worked. Security on this is yet another problem perceived rather than real. The will to introduce a robust and user friendly access system is what is wrong.
We have already discovered that for whatever reason makes sense to Ecot they have discarded the RFID that worked, and we have already discovered that any app based system is unreliable enough to cause many of the 38,000 to simply make alternative arrangements for longer trips.
This needs to be resolved and perhaps the proposed localised WiFi will be an answer. All that I was saying was that inputting a personal ID code at a keypad that has a built in link to the central site would be more robust than reliance on Wifi and a phone talking to each other.