Thanks again Dave and Alan,

I've spent a few more hours on the F/S meter today and still have not found the problem.
I can blow with a drinking straw on any component and there's very little
output change yet with time there's a slow drift with temperature - the output goes up
with increased temperature.

I thought I'd found the problem as by squeezing the ferrite rod 's screening tube
I could get a fair change in the output (about 10 %).
The tube's metal is not aluminium but I guess an aluminium alloy.
In any case I changed it for a proper aluminium tube - no difference !

From the shack's temperature of  23 degrees and taking the F/S meter outside to an ambient
13 degrees the meter's output changed from  0.48 volts to 0.35 volts !!

I have run out of ideas now and am thinking maybe it's even the permeability of the die cast box
that's changing or maybe the perspex end caps of the ferrite rod. . . . I'm getting desperate !

73 Chris G3XIZ