Problem solved!

All those who thought it was something to do with the magnet .... you were on the right track.

There was no magnet in the compressor nut. We know what happened, a few weeks ago some compressor nuts which hadn't yet had magnets fitted got mixed up with a batch of nuts that already had magnets. We thought we'd sorted them all out but one got away! With ready-built engines the fault would have shown up and been corrected here, but since Mark bought a kit it wasn't apparent.

Thanks for your patience, Mark, replacement nut is in the post.

Sara Parish
Wren Turbines