Disco 2 TD5 auto not engaging reverse in high range but works ok in low

So my TD5 auto discovery has started with some fun, a bleeding annoying squeal after the alternator was changed, but most worrying, it only just creeps back a little in reverse in high range. Low range all works ok; and all forward gears appear to be ok though it does seem sluggish. Seems a lot worse when cold (doesn’t seem to work at all when cold), but warm it just about creeps back. Engine is not racing in any gear and other than a little judder seems ok. Any thoughts on where to start?

Start with oil and filter change

+1 on autobox ATF andf filter change. you'll need to drain and refill the ATF more than once to get it fairly clean.
If you are in difficulty getting the ATF into the box, use a plant sprayer, the type with a small drum and a pump, just take the tip off. If you are not already aware, there is a procedure for doing this, post again if unsure.