Found the problem. Opening the Brewflo lid turns off the compressor but it will stay on if you open it slightly. I could put my hand in and I could feel air escaping from the keg collar. It must have been cross threaded. I took the keg out but I couldn't unscrew the collar by myself, it was too tight. So I had my son hold the keg and I managed to unscrew the collar using two hands ..... then Mt Vesuvius erupted! We and the laundry were sprayed with Pacific Ale before I managed to screw the collar back on. After a hasty clean up before my wife came home I reattached the keg and was relieved to see it was still 3/4 full. Turned it on and the compressor ran for a short time then stopped as it should. Beer was good and didn't seem to have lost too much fizz. I've found the keg collars tricky to screw on before (although not as bad as the cap that screws onto the keg liner). In this case the collar must have screwed down enough to seal the cap but not enough to seal the space outside the liner. Will be extra careful in the future!
Those collars from what I've discovered do get easier, some straight off the bat were a little tricky but now they've all been used a number of times they seem to go on without too many hassles at all.
I am waiting for the day some goes crack but until then tighten them up beyond belief