Have you got a flat top sender unit and on old style gauge?...if so it appears from a bit if research that it will indeed work backwords and the solution is to fit a series 3 gauge...
The S2 i'm working on has long speedo cable that snakes round engine bay into dash..iv'e just worked out that the reason is it's got a S3 speedo and original gearbox ,different speedo clip on the S3 cable...