Well I tell you what, folks, all your advice is really paying off now!
After passing my learner permit test on Monday, I had an hour's further training yesterday on a 250cc Suzuki Marauder and enjoyed it so much, I've bought the bike! I felt completely comfortable on the Suzy, it didn't feel heavy at all, and I can get both feet on the ground - which is a huge confidence-booster. Gear changing is still a little jerky but I'll work on that. Key thing is that for the very first time, I felt like a proper motorcyclist - and my dreams of two-wheeled adventures, which just a week ago seemed rather doubtful, have just moved significantly closer!
I take possession of the bike next Tuesday and can't wait to spend every spare moment riding it! Oh and by the way, the bike's registration number starts with "HU..." so it's definitely meant to be!
See you out there, guys...!!!