Brian: Geezer here, and welcome to the group. It looks like you're already on base concerning medieval crossbow bolts. Being close to the continent, you're in a better position to observe large numbers of medieval bolts, but that won't stop me from recording my opinions.
According to Josef Alm, the preferred materials for medieval crossbow bolts seem to be ash, birch, oak, in that order. No doubt when the need arose, medieval fletchers used whatever was available. All the bonafide medieval bolts I have seen were between 1/2 and 3/4 inch diameter (more or less round, sometimes slightly oval in cross-section, thickest up and down)
Ralph Payne-Gallwey shows bolts that are barrelled or tapered, but what I have seen are pretty much cylindrical... perhaps slightly narrower at the head, tapered in width to @ 3/8 in. at the tail. Lengths according to most sources run anywhere from 12 to 18 inches, with the average @ 15 inches. (38 cm) All the fletched bolts I have seen either used wood or leather, and the few bolts I've been able to examine in detail had curved grooves set in the sides of the bolt (inducing spin) with thin wooden vanes inset.
There's no sign of whipping or riveting to keep them in place, so I suspect they were a tight fit and fixed with fish-glue, which actually compares favorably with modern glues, for waterproof and flexibility. Vanes are @ 1/2 in high, often parabolic shape, much like modern archer's feathers, and about 5 inches long.
I had a friend, now departed, alas, who experimented with making wood-fletched bolts of 1/2 in. ash. He built a special jig that allowed a bit of saw blade (version 1) or a small router (version 2) to cut the curved slot for fletching just right for vanes, which were made from 1/16 in. maple, cut to shape, sanded and glued in place... I'm not sure what glue Jim used, but he was mighty fond of superglue (cyano-acrylic) so that's probably it.
He also experimented with forging correct wrapped-socket bodkin points. That takes a bit of practice, and his best were pretty 'bloppy' but given time that he didn't have, he would have managed that too.
Any other questions I can answer? Geezer