Usually the recipient will get a card asking them to pay the excess postage plus £1. Not sure what happens if they don't pay it though.

letter may be destroyed if no payment is received. My optician sent be a letter like this, i was so annoyed that i wasted an hour to get what was an appointment reminder.

it'll get dead lettered within a period of time. i know how you feel though, sent a wedding invitation to my mate in luxembourg, and automatically just put a stamp on it and sent it away... she reckons it'll take about three months to get there on a first class stamp (as opposed to the european stamp and airmail)

the strange thing about paying the excess to get the letter is that you pay in stamps! you stick the value of the "debt" in stamps on this card and then you get the letter! why don't they take money? seems easier.

re: logicalawyer, get your optician to cough up for your time and inconvenience due to their error. maybe they need glasses more ;)

The same thing would happen if you sent something by post but was 'overweight' and hadn't put enough stamps on it.
If you sent a magazine by post and put one 1st class slamp on it and it should cost 40p then the recipient will have to pay the difference plus £1