Yeah, I can imagine this being really solid + private - it's something I'd never have thought of, so ta for that. I'll have a play with these bits next time I'm at Wickes. I guess the thickness needs to be just right for them to overlap by just the right amount.
The other idea I'm keen on is similar to your other suggestion - take some kind of 1" thick, 1' high boards and thicken the ends by nailing some wood on, so they can be slotted firmly into the 2" slots in the concrete posts. Like you say, it might leave gaps, but on the plus side they'll be easy to slide out and replace if any do get damaged, especially where access is awkward right by the spiky hawthorn.
cheers for your help - this problem's been nagging at me since we moved here (this was supposed to be a quiet backwater too :)
G.A.