Thanks, guys. Looks like I have to take onboard more theory, after 35 years of pottering along without. Peter T's book there sounds what I am looking for, though, if I can find a copy.

The problem for me, as I said at the start, is that I'm not very good. Not totally hopeless, just good enough to know just how good I really am, if you understand me.

When I'm playing around with something, because it will likely be something requiring serious learning, jazz or something, I'll come across a requirement for a chord I don't recognise, and if I DO manage to find a suitable sound, I've lost the thread of the tune, and wonder if I'm in the right key still. So I start again, and when I get to that bit, it doesn't sound quite right, so I fumble around again.... Frusrating.
I've bought several expensive songbooks just for ONE number in them, and then found out that I don't like that arrangement.

I do know that I'm trying stuff that is beyond my capacity, but that is Life, isn't it.