I went over to see a client in Strawberry Hill yesterday afternoon, which gave me another excuse to play with Google Maps.
It seems that you can access the routing function in the URL very simply by passing a start address (saddr) and a destination address (daddr). These addesses can be postcodes and they also accept text arguments.
So to generate a route from Coraider (SW19 2RD) to Tighten Up (TW2 5AG);
and the link is:
(If you generate the route from Google's web interface the URL is more complex, but I have found by trial and error that you don't actually need the spn=0.007640,0.023096 and hl=en gubbins.)