After destroying several painstakeingly made cardboard bobbins (when the coil winder got to the ends, the wire being tight pushed the sides out) I hit on the idea of using copper clad board. I thought great, very strong, soldered together nicely, just the job. One big problem though as some of you will be screaming at the screen, yep, it's one big shorted turn, so I "cut" a path through on one side, however as I was taking out the tails/terminations through holes drilled into the board, theses rubbed and thus shorted to each other.
Graham, these lads sell single bobbins or they did -
http://www.ascocomponents.co.uk/ and I got some samples from here a few years ago -
https://www.formers.co.uk/
Andy.