Modern Vespa

I've had to replace the Speedo cable on my lxv125 project bike, and happened to notice that the speed dial actually sits below the rest bar!
Do we thinks it's knackered, or could I spin it around with a square ended piece of the old cable?

I'm thinking knackered, cable driven speedo has a spring that returns needle in ccw rotation to zero, the pin is a stop.

Took a look at the factory manual, nothing on speedo disassembly but you have nothing to lose by taking it apart. If you can get the lens off should be able to lift the needle over the pin. Worst case you buy a new speedo which you were gonna have to do anyway.

I had this happen on my Piaggio Fly. I had to replace the Speedo Cable, but prior to inserting the new cable into the backside of the speedo I used a square driver bit to manually rotate the needle to the resting pin.

The other option would be to separate the gauge from the housing to directly access the gauge face and gently nudge the needle over the resting pin.

I think the hairpin spring inside is broken.
Below a schematic view on how an analogue speedo works.

The cable makes the magnet to turn around and the magnetic field is picked up by the speed cup. It wants to rotate also but the hairpin spring applies a force preventing the speed cup to rotate. It only gets to a certain degree depending on the speed of rotation of the magnet. The needle is fixed to the speed cup and the angle the speed cup gets gives you an indication of the speed you drive.
On slowing down the hairpin spring brings the needle back to its zero location where the stopper is.

If the hairpin spring is broken then the speed cup will start to make a turn until the needle hits the stopper from the wrong side and nothing is there to bring the needle back to its correct zero position.

If you turn the magnet in the opposite direction, you will get the needle back to zero, but as soon as you start driving it will be back to the position it is now.

I doubt these speedo's are designed for repairability, probably it is completely sealed.
I am afraid you will need a new speedo.

Generally the bezel is crimped on to the body of the speedo & lens
Take a cut off wheel, cut the housing in half
Make sure to leave enough material on either side of the cut to make it easy to to hot glue or other repair , should you be able to repair

I used this method to repair yam xs650 speedos, which would shake the needle off
I'd paint toothpick with florescent paint & jb weld that to the base

Thanks for all the comments. Most appear to be confirming my view that, it's knackered!
I will try moving it back manually with a square drive, but don't hold much hope that it will fix it properly.
Did try finding a way to break the unit open, but it is pretty much designed not to be tampered with for obvious reasons.
Looks like it will be a replacement which is about £250 from what I have seen…

So this story ended as expected! Old Speedo was dead, so replaced it along with the cable and gear too.
Hopefully that's the most expensive part I'll need to buy…?! Should've doubled the value of my Vespa!

Especially with only 7/10's of a mile on the odometer! It's as new!

Glad you had a successful result.