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Thank you -I spotted it on eBay but I don't think it will work as there are no tab/notch in the fseatpost bolt hole to hold it in place
I think what I need is more one like this:
https://www.ebay.co.uk/itm/Vintage-Pete-Matthews-seatpost-bolt/163628349525with an external part that can be held with a spanner while the allen part bolts through.
Ceeway has some serrated ones that could work too:
https://www.framebuilding.com/Sundry-Components.htm
One of these? https://rover.ebay.com/rover/0/0/0?mpre=https%3A%2F%2Fwww.ebay.co.uk%2Fulk%2Fitm%2F332176582277