Aha then I think my theory is right - I have the +ve's tied to XN on the BoB, not the -ve's.

Going to rewire now and hopefully having them the wrong way round the first time hasn't damaged the drivers. A classic Meccano moment if you've ever tried to build that stuff as a kid from the instructions (they deliberately put errors in their diagrams to make you think for yourself ;) ).

I'll report back in an hour or so.

Cheers


Chris