lots of useful feedback - thanks guys.
I'm going to go with the making-a-guide-from-a-piece-of-scrap technique,
plus some trial runs and slow drilling.

As someone mentioned it, I considered tapped brass round for the nuts
but decided I couldn't be sure of locating the hole for them perfectly
aligned with the bolt (though it must be possible as plenty of others
take this option) and instead made some long nuts out of ~8x18mm steel flat.

graham.