Congrats Iain.
I agree with Aerbabe - do as much planning on the GND as possible. I have done so much manual nav work that I would be scared to use a GPS for anything other than checking I hadn't in advertantly strayed into a MATZ or FIS. Apart from (in my opinion) being lazy, using the GPS -D-> (Direct To) function seems pointless unless you are in unrestricted/controlled airspace because sometimes there are so many things to avoid like danger zones and obstructions.
I had to get a position check from Bristol ATC when I was on my QXC because I was told to join the zone at Radstock VRP instead of my planned Chew Valley VRP. When I thought I was overhead the right palce, the controller came back telling me I was overhead Bath! Everyone makes silly mistakes from time to time. But expect the unexpected even if you have done your planning on the ground, you, or someone else may change it for you en route.
Good luck with the rest of your flying.
Regards, Trislander