This may or may not work. Has worked for me a few times:
Plug a large dimmer-equipped lamp into the same outlet as the valve timer transformer. Set the lamp to about half dim. Take a portable AM radio, tune it to an empty frequency near the lower end, 500kHz-700kHz or so. Hold the radio near the cable that goes to a valve, see if you hear the rat-tat-tat noise. Try with the valve on and off. Find the direction of the radio where that noise becomes loudest. With some luck you can then follow the line unless it's buried too deep in the ground. If you can hear it follow the trail until it stops, that's where your valve should be.
CFL (energy saver bulbs) might work as well, they also create radio noise.
Regards, Joerg