Update.
It seems that I have now developed something called a "Swan Neck Deformity" where the joint nearest the knuckle is hyperextended causing the last joint to be unable to straighten. It's slight at the moment but needs to be fixed before it gets worse.
I'm in this extended cast for a week. It has a rod along the underside of the finder to help it stay in shape, then next week I get another new splint made that will keep my finger in a "slightly odd" position to try to fix it.
I am getting quite bored with this.