In reply to mypyrex:
Strangely had this on Saturday but got a grey card saying that the sender hadn't paid the correct postage and could I place £1.12 (12p in underpaid postage +£1 surcharge/admin fee) in stamps on the reverse of the card and RM would then redeliver. I guess that if you don't pay it'll sit at the delivery office for a few weeks before being returned to sender or if you go down there and tell them to RTS then it would have ben better to just pay up than have wasted the time either way inconvenience!
Anyway according to Google you appear to have been lucky...
Sorry UKC won't allow me to add two web links because it says "Please don't write verylongwordswithoutanyspaces"
GO FIGURE!