No reverse gear

[DentedDevon]

I have an itermittent problem where I cannot select reverse gear.
It will go in with no problem and then , usually when its been stood for a few days it wont.
Take it for a drive 10 miles or so and its fine again.
Its an ABD gearbox.
Ive changed the gearbox oil and as it works ok some times I dont think the problem is with the linkage.
When it wont go into reverse the selector arm on the gearbox wont go in any further than the 1st/2nd position and dosnt put the reverse lights on.
Ive tried taking the reverse light switch out but this makes no difference.