Hi there, Stuart and Ksor,
As you may know, Stanley make at least two different sizes of the Yankee Screwdriver (I can't remember the model numbers off-hand).
The bits aren't transferable from one size Yankee to another. I have the largest model and still find it very useful for some jobs alongside my cordless electric screw-drivers. I have several different sizes of bits for straight-slotted screw heads as well as both Philips and Pozidrive bits in various sizes. You should be able to get extra bits from any good tool-supplier who stocks the Stanley range.
Usual disclaimer - no connection with Stanley except as a satisfied user. (Well, except that I'm most DISsatisfied that I can't buy Stanley Screwsinks any more!!!)