Well done rjtjrt - that Plane Finder site is a good get, but it doesn't show the object of my interest.
Given the likely speed involved it must have been something military and perhaps relatively stealthy. I'm obviously wrong about the time - maybe it was 30 or 40 seconds to traverse 90 degrees.
So it was possibly something to do with this:
US Navy EA-18G Growler visit - Royal Australian Air Force - I wonder if one popped down to East Sale yesterday?