Hmmm, so the FSM is all working well… up close. You see, the article did say that if an analogue meter is used (as opposed to a digital multi-meter) that the sensitivity would be reduced due to the high impedance of analogue meters (I think the one I used has a resistance of 3k or 30k Ohm). I was happy to accept this and see what I could see.
Well, the result is that if I move my hand-held @5W more than about 1m away from the FSM then it barely detects anything! Not quite what I'm after as it will probably not be practical for testing 2m yagis up on poles and the like. Further, it also doesn't seem to detect 70cm signals at all!
So, it looks like I'll be looking into adding a simple transistor amplifier to it – good chance to put some more theory to use.
(That said, an amplifier is one of the key differences between the design I built and that described in the Radio Theory Handboook I mentioned.)