Following up my debacle at the Post Office I checked my bank and credit card statements when I got home. Payments appeared to have been taken from both. spent a long time explaining the situation to both the PO helpline and then a very helpful chap at Thetford who said the money will only have gone into a holding account and will be returned when they balance up and realise the currency was paid for twice. It wasn't.
I was referred back to the helpline to get my refund, but spoke to someone who patently didn't believe me and was trying to get me to ring the branch again...but got that sorted. now looks as though I'm going to have to post a whole load of stuff to them to get the refund! still need to actually speak to someone again to find out what they want. Hmpf.