23.8mm clamp. My brother had one on his track iron for riding to the track; with traditional bars on which the 26/26.4mm section is a sleeve rather than a bulge you just have to leave the last inch of the thin part of the bar untaped and clamp to that rather than the centre sleeve.
@Dammit found it! here you go, Dia Compe 131 Road Brake Levers;
https://www.velovitality.co.uk/collections/brake-levers/products/dia-compe-131-road-brake-levers
My advice is to replace the screw bolt with an allen key bolt which make removing/installing a breeze at Herne Hill.