I took it out and it's working fine. Used speedo app to confirm speeds.

Since you have removed it yourself etc there will no doubt be evidence of tampering thus I suspect you will not get any warranty repair work done

I think the OP's statement means he took the car out to see if the speedo malfunctioned, rather than he took the speedo out?

"Do we need to register the fault with the dealer so if it happens again out of warranty, it'll get fixed ?"

The garage will no doubt wish to investigate before registering a fault. We could all make a simple phone call to prolong the warranty on many items if this dodge worked.

Since you have removed it yourself etc there will no doubt be evidence of tampering thus I suspect you will not get any warranty repair work done and will probably be charged for the time taken to dismantle the dash.

But why on earth did you initially take to car to the local garage and then tamper with it yourself despite being told by the local garage to take it for a warranty repair.

Its what warranties are for.

I think he means he took the car out for a drive...

Its a Fiat so probably a bad connection or failing sensor. If taken to a dealer they might charge a 'diagnosis' fee if under warranty and no obvious fault is visible.