Thanks John, had a look at that web site, and it didn't clear the fog from my brain :confused:, but then I found http://www.automationcomponents.co.uk/ things are making sense now :).

As I understand it (using Marchant Dice codes as an example) a TR16-8P4 screw is 16mm in diameter, would move 8mm per revolution, and has 4 starts. Anyone able to confirm this for me?

Also MD don't seem to sell multistart nuts at all?! seems a bit strange.