Well, I've bought it and it looks original to me! Thanks for the information, Dwsi. I thought that there might have been some of you bidding against me but there was only one other bid.
I've scanned the document as well as I can but my scanner's only A4 while the document is foolscap and is sewn together. As Fester says, it's an historic document so I don't want to bend or fold it too much.
I'm not sure how well these will come out and I do want you to be able to read the document, so let me know if you've got any suggestions for improving the quality.