Thanks to both of you.
Another question. I have looked on Ebay and there seems to be a few passes available. One particular seller from Manchester has quite a lot at £6.99. However most of the UK sellers have passes without serial numbers on them. They look official but without the serial numbers which seem to be on the US sellers passes. THere are also passes in Black and white.
Are all of these valid?
BRobson - You said that you are just back and used one. Did your pass have a serial number on it?
For less than $20 (about £10) I think it's worth a try. I bought one from the US with serial number on it and instructions about how best to use it.
I don't think that there is any way for them to tell it's been re-sold so it should be ok. I cannot imagine a Disney parking attendant vigourously checking them anyway, but I could be wrong?