After getting back from the lakes on wednesday I put the information from my Garmin onto the computer. Since then I've been zooming in on google earth images and working out where I went wrong. Hopefully next week I'll try another new route to practice my navigation.
As can be seen from the picture my Garmin has missed all the twisting I did on the path and just drawn a straight line. This means I have no idea how far I actually, rode/pushed/carried my bike. I've never had any problems with satellite pickup before but I wonder if in the lakes it struggled?