We have an extensive, professionally installed, garden lighting system
which has recently started tripping its dedicated RCD. I have
eliminated all the bulbs, luminaires and accessible cables which,
sadly, means the problem is almost certainly in one of the many
'waterproof' underground connection boxes. How on earth does one
trace a fault like this? Dig up the entire garden?

Richard.
http://www.rtrussell.co.uk/