Chris, you can buy most of the Preston attachments, drill out the existing rivet and fit a new Rive D36 knuckle, and then re-rivet it back on using a 4mm dia, 10mm long rivet.
Fosters and Nathans of Derby both sell the knuckles (£5ea), which are available to accept either the square or octagonal attachments. The octagonal attachment can be bored out with a 12mm drill bit to make it round, this will then be a tight snug fit for the round Preston attachments. But most people buy the D36 knuckle with the square attachment which is slightly easier to fit but not as tight.
Hope this is of some help.