Your Garmin help, please

Nope. I have been following the course for maybe 5 miles or so (starting from the beginning) in town and the pink line decides there is a short cut. It usually joins up with what I wanted a short distance afterwards, but includes some really risky turns. I have re-route witched off. Do you lot not get this? I have some routes on my MapMyRide account i you fancy taking a look:

http://www.mapmyride.com/routes/view/185531360

I have looked at the gpx files for two of your courses.
Your Battersea! commute is 15 miles long and has 50 via points.
Your HH to home is 9 miles long and has 1550 via points.
When you load a gpx to your 800 it recalculates and routes you thru your via points.
I suspect the issue you are having is due to having too few via points therefore the 800 is sending you the way it wants rather than your chosen route. Maybe!!!!