Thanks Rick for your useful response which focuses on the erratic nature of the fault. Regarding the diaphragm assembly in the lens, this would be a good reason if I was only using the Sigma lens.
Unfortunately the fault is also present with the few interior shots I took during this period. In particular I took 4 closeups of panels on a font with my Canon EF 50mm 1:1.8 II lens. The shutter speed would probably have been down to a 1/15th using the P mode.and this lens usually gives me excellent results. Any further comments would be appreciated.
ALSO help in locating a camera repair shop would be welcome. My post code is CR0 9BE croydon.
Thanks from Ken
Here's a link to Canons authorized repair site :