I should have added my speedometer is a SMITHS clock which pretends to be an old mechanical chronometer by jumping up and down by a couple of mph at a time as you accelerate and decelerate.
My speedo (2013 +4) had the same jumpy needle from new and developed increasingly erratic behaviour such as reading a constant random number of mph under or over the actual speed, typically 20-30 mph in one direction or the other.
It was fixed under warranty and since then for 2,000 miles has been rock steady without any odd behaviour at all. I assume they replaced the speedo because when the car was returned the odometer was reset to zero.
