OK, how did you pinpoint it to actually the speedo itself at fault?
As wraithman and shadow11 says next point would be gearbox feed. I understand there is a toothed spindle which can be a problem and replaced.
Re the aerial it is brand new unused and sold to me for my shadow 1 but it definitely is not for the SS1. Sat on my shelf for over 20 years till i finally came to fit it then discovered it was for a later car. I am not sure at what year the change was made maybe when the SS did a lot of electrical upgrades around 1975.
Problem is the original on mine was a screw in connection under the wing to the aerial post and the later one just a push in fly lead from the post so would have meant finding a female mating part to put on my lead from the inside of the car or replace the lead which seemed a lot of work so I decided to fix my orginal. I got a faulty one with a good shaft and managed to do a change over and it wthen worked OK. The old one used to just stick half way up or down despite the extension pieces were not bent or damaged it just had to be pulled up or down manually when trying to press switch at same time.
The one I have is genuine boxed RR part number UD 21483.
I paid £120+vat over 20 years ago and as the modern Hirschmann replacements sold as patterned parts by the dealers at around £120 I don't see it will expect to get much different unless from a concours fanatic.
The main advantage it is the type that requires the remote up/down switch rather than having to wire in feeds from the modern aerials that are automatic from switching on radio.
As for the replacement from dealers not sure why so expensive as I have several new boxed electric aerials some of the switched type and some automatic and none have usually cost me more than about £30-£40........but they do not have the flat topped outer like fitted on the RR.
What is wrong with your aerial as the outer shaft if bent is worst to replace as I did from one with a faulty motor to fix mine.
The motors/gears can be got at to clean commutators and re grease a bit easier but in all honesty the most work is getting to the aerial as wheel arch liners have to be removed.
Just that again first check is the switch itself.
I will add some pictures later when it is daylight as I still have the old parts.
All the best