Spider Speedo Problem

Help please : random speedo on our Spider today. Just noticed that the speedometer started showing random speeds, or no speed at all! The odometer also only racked up 4 miles after about 20 miles of driving. Occasionally the speed was displayed correctly for a few seconds, before switching 0->5->23->3->65->0 and like that for the whole drive. Stopping to switch off the engine and on again didn't clear the fault, which I'm guessing is a wiring or sensor problem, but I've no idea where to look, as I don't know where the speed sensor is. Any help appreciated. Thanks, Ian

I believe the speed sensor is an ABS sensor on one of the front wheels, so maybe start there and work back?