I bought one of those about 12 months ago and it seems to work fine (about 10% optimistic with low-profile tire). Its a straight fit for the cable but a new mounting plate will need to be made-up because the repop has the cable entering in the center, not off-set.
http://www.ebay.com/itm/Smiths-Replica-Speedo-Meter-Speedometer-0-80-MPH-Black-For-BSA-Brass-/322798925644?hash=item4b2850b74c:g:wTMAAOSwEW5Zet84

email (option): cas.vanderwoude@gmail.com

That's very encouraging John. I think I will contact them tomorrow and see what they say. I would much rather keep the original if possible. Having now managed to open it without breaking the glass or damaging the bezel I can see that one wheel is partially stripped but everything does actually move and most of the mechanism seems to be in reasonable condition. May not be the mountain of cost I was expecting after all. Thanks for your reply. Mike

email (option): Paduano mike58@gmail.com

No problem' Russell will do you a repair to get it working for a really good price, He got my B31 speedo working great a few years ago for £45 and it's still going strong' Regards John.

email (option): rustytomm20@hotmail.com

Based on your recommendation I contacted Russell today and have arranged to drop the offending speedo off. Prices seem really keen and I was very impressed when I spoke to him on the phone. I will post the results once the work is done. Mike.

email (option): Paduano mike58@gmail.com