Roger - good chance that a circlip on the mainshaft has broken up or moved, this allows the synchro hubs and the gear train to move in one direction along the mainshaft, this would also account for the non meshing noises..
Believe me - there's nothing to complicated about Ford gearboxes, buy a good Haynes manual and get stuck into it, you can make up a dummy layshaft with a bit of old pipe and provided you can access a good vice, ordinary handtools will do everything else.
Blatting around Bucks in an R400 - at least I will be when the weather improves !