Ah, thanks for that guys, I did just send PWT an e-mail asking for a refund of the extra cost but if it's the fault of Royal Mail (which is NOT surprising) then all's well. What a stupid, stupid system. I'll still wait to see what they say, since the note said it wasn't a customs charge but was the fault of the sender, just want this all cleared up and done with! Because if I remember rightly I did buy the $35(ish?) ukulelehead bundle, and that came through with no problem, no extra charge, but I guess that's near the £18 limit considering the exchange rate?
Ordering things from overseas is just a pain in the ass in general, so I'm always apprehensive/pissed when things go wrong.