Geoff - I've scanned in 12" LP covers on an A4 scanner in two passes. The secret, as far as I can tell, is to (a) make sure the lid is firmly down on the cover so as to avoid distortion in the scanned image - no easy if the scanner has any kind of a lip, and (b) use reasonably good software to marry the images. I use Photoshop myself, and it's fairly good.

An alternative is also to see if the cover is available on the net as an image which you can download for private purposes.

Mind you, A3 printers have dropped in price - any idea what an A3 scanner might be these days?