Yeah i have read about those, the thing i like about the welbike is that i can keep it in a shed and it can fit in a car boot. I'm not sure about the Flying Fleas but a lot of the engine parts for a welbike, for example:Air cleaner, are standard on a lot of villiers lawnmowers and autocycles, the only diference i can find is that the welbike parts are often brass but not on other villiers engines.
I took a look at your book on the internet, looks pretty good, i may have to save up some money.
Good luck to you with any current or future restorations
Thanks
Martyn