When I got done by Parking Eye for going over the time by 14 minutes we had two cars with us and only one of us got the letter (who do you reckon!). For me it took them 5 days to get the letter out, I'd say you are fine after then and I don't think they can go for you legally after 14 days anyway. I think they must get that many that they can't physically process enough of them in time..
I appealed mine on the basis that if they could provide me evidence of the loss which justifes their charge in a free empty car park at 10 at night then I would happily reimburse but otherwise I would be going to appeal through POPLA (costs them £30 just to hear it). Recieved it on the 15th of November and still awaiting to hear off my appeal to them, I am even thinking of charging them for replying late.
In my mind they make their money off the people who either pay up or ignore it, it costs them money however to fight people.