I've just had a message from the seller, she is in France, saying she has posted my item today but the postage had gone up in January (she was supposed to have posted end of December anyway but that's another story) so can I please wire her an extra £4 
I've just responded with the following but now worry I was a bit harsh, it could be a language barrier after all! What do you think?
"No, I would not, I frequently sell on ebay myself and wouldn't dream of asking for extra money after a sale was made! I have in the past had to pay over the odds for postage when I have got it wrong as a seller, it is most definitely not up to the buyer to correct mistakes on the seller's behalf. I took the postage into account when I originally made my bid and went up to the amount I was willing to pay. I feel you are a very inexperienced ebayer? In all my years on here I've never encountered such a request!"