[Uncle Bulgaria]

Once again you demonstrate the gift of being able to explain in terms that the tyro can grasp. I think I understand what's going on here now. Another of your posts I've printed to put in my 'references' folder!

I'm populating the board, and was for most of last night with the company of Miss Marple. I haven't 'done' PCBs before and putting all the resistors in the right holes was strangely meditative.

I decided the turns must have been shorted, and rewound the secondary (very roughly). This time, I used one spacer (0.05mm) and got a result of 4167nH per turn. This sounds much more reasonable.

I agree, the fact that a page of instructions on how to wind the thing doesn't include the basic specifications of the core or the rationale behind the winding is pretty annoying. It's taken the previous thread and the contribution of people who knew ferrites well enough to propose datasheets for the core the Jaycar part was based on to enable me to get to this point. I'm very grateful!