The bench is finished! I am very pleased with the holdfasts, they were made by Simon James and I got mine from Classic Hand Tools, delivered next day at the very reasonable price of £49 for the pair including delivery.
Some people have trouble using holdfasts on thin tops, I experimented on the front apron (just over 40mm thick) and the holdfast need only a light tap to grip tight. I tried them in the top of my old bench which is 38mm and they worked there too, but it sometimes took a couple of firm taps to get them to hold. The holdfasts are very well made and have a rough finish which helps them to get a hold even if the top is a bit thinner than optimal..
I will put some holes in the top too, but haven’t decided where to put them yet.