OK - considering the input received, I've purchased the wheel centers (black ones, as IMHO it would be 'a bit much' to have coloured ones on the wheels AND the nose), as well as these
http://www.ebay.com/itm/331353521131 bolts on the theory that if 26mm works with steel wheels, 28mm will be fine with the alloys. (yes, I'll be swapping out the on-board lug wrench for a BMW piece in 17mm). I had considered getting just the bolt caps, but have found them to be a PIA to deal with in the past.
This operation will also provide an opportunity to properly torque the lugs, which may have contributed to warping the old discs (not sure of any such history with the M100, but it has certainly happened with other models when the garage guy over-applied the impact wrench).
Photos will appear in the gallery when "fully dressed" - centers will take a bit of time to get here from Portugal!
Thanks to all,
Peter