Stu, as Dave H mentioned on the M20 forum, the combination of later WD left-hand mounted speedo head and 1939 civvy front wheel is likely to cause you cable-routing problems. I'd be inclined to make up an early WD / pre-war style centrally mounted speedo bracket which will let you run the cable straight down the right hand side.
By the way, well-done on needling the oh-so-superior M20 owners over their choice of machinery. They need taking down a peg or two from time to time !
I thought i would post a couple of pics of my finished bike!
A big thank you to everyone on this forum for their guidance, help and parts, in particular Rik who took time out to send me lots of information throughout the process!
And of course Rob, who runs this fantastic site!