Thanks Hank. yeah I got the basics, although I understood your explanation better. I was looking for a starting point for the magnatising coil, as you said there are no values given.

The main inductor bit is easy, its about 550 turns of 38AWG wire on a pot core. finished size is about the same as a stack of 3 £1 coins.....

As I said I planned to look into this later (too many projects) but Ive got side tracked when I saw this thread about coil winding..

May have to pick your brains when I get round to it.