Thanks for replying.
Yes, I gave them my full address.
It works like this: you park the car, then start the session, a £50 deposit is taken from your account and it charges until the battery is full. I wasn't planning to charge it to full - I just wanted to charge while I was in the shop for a few minutes. That's what I always do, and it has always worked.
I haven't contacted Total yet, but I'll send them an email about this shortly.
I checked on the Merton website, and the PCN status is "On hold"...