Hi gav, yes mate, they are available, but as these parts are no longer manufactured by Rover-Scott Bonnar, they are after-market, and somewhat expensive.
The nylon drum gear is now machined from metal, (which it should have been in the first place) and the pinion the same as the original.
I am sure Bruce will source them for you and put them up in the online store.
Gav, a big warm welcome to the forum, I hope we can help you with any probs you may have, as we all love to learn new things and tinker here.