I understood that the IRIS aircraft operated by RAF Signals Command, carried out calibration of instruments and communications systems on RAF airfields. Here is at Changi in November 1961, presumably in that role. I think she made an annual visit?
Signals Command Hastings WJ338 'Iris III' at Changi November 1961.
David
david taylor images ......look for the 'Aviation' gallery for the Singapore images.