Yeah I'll have your old one........I'm busy collecting them as Steve has noticed

Following on from what modulator said, better still, disconnect your linkages at the box........its dead easy, one wire clip and 2 plastic clips. Now move the gearbox selector lever into reverse on the gearbox itself. As you look at it I'm pretty sure (although I may be wrong and its 1am so I don't intend to go out and verify) its pull towards you and then right.........you should hear that distinctive reverse gear sound as it engages. Try it a few times..........nice and positive? Then its your linkage out of adjustment.......if not see me.......reverse gear should be a piece of p!ss to fix and I have plenty of spares.