Axminster still sell these for the Miller dowel system. Three different sizes. Might be worth checking the sizes with them to see if any of them would do the job:
Miller Dowel Stepped Drill Bits - Dowelling - Biscuits, Patches & Dowelling - Fixing, Gluing & Finishing | Axminster.co.uk
The dns-twister pen kits that Phil Irons used to sell also used a stepped drill. They seem to have disappeared too.