After about 2 hrs of dilly dallying – the new tach sensor installation is done as advised and am very happy now. One monkey 
off the back and now onto the next – thank you for advise from William Wynne of Flycorvair.net
The Tach sensor is from William and it is set to sense the 2 AN3 bolts drilled into Toothed Flywheel 180 deg apart.
