I bought a multimeter and found a dodgy connection – inside maplin's battery holder!
The new phototransistor didn't work but an old one I had was perfect.
I have measured the various shutter speeds of my camera. Have to say they are pretty much spot on at least within the limits of the measuring system.
Just from random samples – 1/41, 1/59 and 1/111 (for 18, 25 and 54 fps). Pretty close to the spec.
That's a good result. Thank-you for all your comments and help. Back to film tests with confidence in the camera system!