Update on what I’ve found.
I ditched the XLR-RCA converter and got a mono ¼” plug to RCA socket converter. This was plugged into the MIC100 and then an RCA cable to the PC. Disappointingly the result was no different to the first graph.
I did note that using this method the “Gain” and “Output” dials on the MIC100 both had to be turned to their maximum setting to get a usable reading, which I suspect is not normal.
Next attempt was to swap the MIC100 for an ART Micromix, again using the ¼” plug to RCA socket converter. Instant success. I only did a quick uncalibrated test to see what the shape of the resulting graph would be, and it looks much like expected, with a largely flat response. The massive drop off has gone.
I think that either
a. There is an incompatibility somewhere with the MIC100, but I have no idea what it could be.
b. I have a faulty MIC100
Thanks for your suggestions, all I need now is to get the house to myself for a morning to do some proper measuring.