Dual display not working after firmware update (July 2015)

27" iMac 10.9.5

I've been using an external display (mini-display to VGA) ever since I got the iMac a year or two ago. It was working this morning but after I applied the latest firmware/software updates it's no longer recognised. No cables have been disturbed and nothing else happened as far as I can see.

Any ideas?

Thanks