Picked my car up from dealer yesterday who replaced all 6 injectors - problem solved
. Top marks to dealership (BMW High Wycombe) for taking it seriously and resolving under warranty.
It is still noisy on startup (as are all B58's) but it doesn't make that tapping / clacking noise now.
Master technician updated the software (not sure what modules) but all my custom settings gone. Gear change seems smoother, and cold start definitely different but other than that all good so far.