Andy....as Eric says, it's a collet chuck. there's a cone shaped piece that, by moving inwards or outwards, forces the chuck jaws to expand or contract. The body of the chuck, and a ring that fits onto it, are threaded together, such that when you tighten or slacken them off, they shift the cone in or out and the expand or contract. You would normally have two 'C' spanners, but I see that there's only one there. Also, there's a lot of metal parts rubbing against each other, so they have to be kept very clean, otherwise they jam and ca be the devil's own work to free up.
It's possible that the maker was Peter Child, so it might be worth mailing the pictures to them and asking them for advise. I have always found them to be remarkably helpful
Les