Had a look on the Triumph forum , a common problem seems to be the splined circular magnet which sends a pulse up the cable to the speedo , this has been known to crack/break resulting in a loss of input to the speedo .
This guy , Trevor is near Warminster , have bought a few parts from him and stocks the part , if it turns out to be that .
https://www.triumphparts.gbr.cc/product.php/6806138/