Speedo not working

I have spent the last few days of my Xmas break fitting a new rear disc to my 796. Completed the job and have just been out on a shakedown run to find the Speedo is not working and the odometer is not going up.

Is this because the OE rear disc somehow activates the rear wheel speed sensor to create the feed for the speedo? This is one of the early non ABS versions.

Any one got an idea?

Many thanks