The local dealer says this is normal (the inside of the front tyres were bald) - geometry tests showed everthing in alignment.
I asked the dealer if I could replace the wheels with another size and with less expensive tyres - they could not say! Anybody out there have any ideas?
20k on a tyre is not too bad. £740 for 4 is expensive.
On BMW's the rears tend to wear in the middle (even if inflated correctly) and the fronts (especially the NHS (left) in the UK (roundabouts)) on the outside edge. Wearing on the inside edge is indicative of incorrect alignment.
On BMW's the rears tend to wear fastest, so when they get low in the middle put them on the front where they will go fastest on the edges and put new ones on the edge.
If the fronts wear very asymmetrically you can turn them one the rim (if reversible) or turn them on the rim and swap sides (if not), but I would look for a geometry fault first.