Sluggish autobox reverse

[gdr]

My W reg E39 535i sport auto is a bit slow to take up reverse after it has been sitting a few days. This seems to have got worse recently. No problems going forwards, but reverse unable to overcome gravel on my drive for a minute or so after starting engine. Revving engine no help. When warm no problems.
Any ideas before I take it to dealer and face inevitable ££££?