I can also confirm that not all Mk III had spoilers. I did my Proficiency with 637 GS at RAF Gaydon in 1969 and all (5?) did not have them fitted - no use really on a 2.5 mile runway!
However when we transferred to RAF Little Rissington in 1978 we were re-equipped with spoilered MK IIIs.
I seem to recall being told by a MGSP chap that the spoilers were fitted during Major Servicing - oh what fun to create asymmetric drag from the back seat by pulling one of the cables!