Thanks for the suggestions folks. I'm really not interested in 'correct' positions etc., just the way that works for me. And by that I mean the method that enables me to do what I'd like to with the maximum effectiveness and minimum discomfort. The reason I'm uncomfortable with the method that comes naturally (the bent middle joint) is because it restricts my flexibility as a player. Conversely, when I play with a thumbpick I am able to use a straight thumb, which affords me more versatility (mainly to dampen the strings occasionally).
I stumbled into a music shop yesterday and got chatting with the guy there. I tried on one of those white Jim Dunlop thumb picks. It was a large but he told me that referred to the pick size rather than the loop, which surprised me as I always thought it was the loop, but he was right as the small ones had smaller tips. He told me to put it in hot water to adjust the size, which I have done before with mixed results, but this one is perfect and so I've decided to buy a few of those, make the necessary adjustments and then see how I get on with them.
I've settled on this route because it gives the flexibility of a personaly preferred hand position (thumb parallel to the strings), which I am willing to 'trade' for the naturalness of no pick etc. as it feels like a fair exchange to me.
Fingers (and thumbs!) crossed
