Bose: "Contacting the examiners may be one way but it one hell of an exercise to reconcile the examiners records with the CAA records and one could ask if there is motivation on the part of the CAA to that sort of exercise."
Why would you or the CAA need to do any reconciliation? It would be a very simple matter for either the CAA or AOPA to contact the relatively small number (guess: around 150?) of FEs and get accurate info about IMC tests; a voluntary survey will take longer and is unlikely to produce accurate data.
Presumably all you need to know is the date and CAA reference number for all IMC tests performed over the last 2-3 years.
The only thing an FE survey wouldn't catch are the people using UK CPL privileges, but your proposed survey is unlikely to catch them either.
HFD