Here's a link to the circuit that I am thinking of using
http://www.pan-tex.net/usr/r/receivers/ssmeter.htm. However, I'm not sure that the polarity of the diode protecting the FET gate is correctly shown. Since AGC is negative (in my case about 0v to -6.8v) surely the diode as drawn would simply short circuit the AGC line? Shouldn't it be the other way round, or am I missing something?