The box has covered about 280,000 miles and works fine, apart from the overdrive which is now showing its age. It seems as if it is suffering from reduced internal oil pressure. The symptom is occasional reluctance to engage below about 45mph. It is worse going downhill, I assume because the oil drains to the front of the box.
Strangely, in view of the declutch argument, it works much better if it has some resistance to bite on, ie if I don't declutch but keep my foot on the gas.

My questions are
1) Is it straightforward to unbolt the O/d and fit a replacement leaving the gearbox in the car?
2) Is there any point in trying to do any in-situ overhaul by unscrewing the valve plugs and replacing the o-rings and seals? Or should I just accept it needs replacing or a proper overhaul?

Thanks if you can advise