Hi Colin:
Have you thought about the Hustler 5 or 6 BTV vertical?
I think the 5 is only 6 or 7 feet taller then the diamond and would probably perform better.
I would suspect that the diamond would be much like the Hy-gain 18-AV which does well from 20/10 but OK on 40 and not very well on 80.
The only reason I didn't get the Hustler was because I didn't have the tools or sited assistance who could properly tune the traps and my XYL, who isn't a ham didn't think she could do it either, not to say that I'm not pleased with my 43 footer because I love it. LOL
I would also look at Zero Five in fact to see if they have a vertical that's in your height restriction, if this is the reason for the diamond.
Back to the Hustler for a moment, I think the price point is probably equal to the Diamond your looking at. I think the 5BTV sells for 250 at Radio World.
73:
Mike VO1AX