Put laptop back on its own aerial and system, and the inSSIDer worked straight away.
Fascinating!
My signal on the active router seemed very steady, while my nearer and quiescent unit had its output bobbing about over several db. Odd. It's almost line of sight in a closed down workshop with plastic roof.
Despite few other users showing, the signal was switching between poor-fair-good in sharp square-wave jumps. Yes I understand sampling rates, but this just had the feel of some external influence.
Again, inSSIDer did not function with the external USB devices being a boosted receiver/aerial.