reluctance to go into reverse when warm

[LSW41N]

Lots of threads on gearbox's and good info on Wiki, but I can't find anything that matches my problem:

1.9 DG engine, 4 speed box in a Autosleeper High Top. When I begin my journey, ALL gears change fine. After warming up (10 -15 mins of driving), still no probs with forward gears but there is a real grind and reluctance going into reverse. I've started by looking at gearbox oil - it's clean and plentiful, but I might change it anyway. Should I look at the magnetic drain plug for the 'christmas tree'? And if I find the tree (wear and tear?), what next? What other things should I look at / consider?

[LSW41N]

Any thoughts before I head off to Bus Types?!!

[syncronized]

Sounds like the clutch is dragging, hence the "grind" when trying to put into reverse. Might just need bleeding if you are lucky