Many thanks Lynx815, my date of “circa 1977” was a guess, but fitted with the other photos in the box of slides that I was given. 

I’ve just had a close look at the online photos of WF128, currently preserved with NASAM, and there is indeed a visible plate on the lower fuselage below the cockpit. This plate matches the location of the large aerial in the slide photo.  Also the serial WF128 is close to what little can be made out on the slide photo.  You have also shown that WF128 didn’t serve with all the other Sea Princes of 750 Squadron, which is why contemporary photos of that squadron only show day-glo aircraft.  And finally if WF128 was with Belfast Station Flight, possibly on ‘Troubles’ related duties (the extra aerials), then that would explain why it was elusive to the cameras of mainland Britain.

Result!  I can now caption the photo as WF128, dated 1975, Belfast SF/MoD(PE).

Many thanks again.