The box sounds normal TBH. There are lots of comments about the box, try googling "F40 gearbox", one suggested solution has been to reduce the amount of oil in the box, but not too sure about that one!
I've replaced the reversing switch twice on mine, it is located almost under the battery tray. you can test the switch by linking the two terminals on the lead, just make sure it is in reverse - don't laugh it has been known!
If it is the switch, it is possible to replace it without removing the tray, the cut-out is just large enough to get a spanner - I used one of Halfords ratchet combination spanners, the type with a 17mm (don't quote me on that) flexible ring (if you know what I mean), and another spanner across the open ended jaws for leverage. I got my switch from Eurocar Parts.