The garage electricity supply had been unusable for a while; there appeared to be a fault somewhere on the supply cable (which was only protected by some discontinuous piping) causing it to frequently short out and trip the circuit breaker.
A new armoured cable has now been laid and I've replaced the customer unit. I now have working lights in the garage and the garden with plenty of scope for future electricity use up to 63A... there are now extra sockets, including some outside for the lawn mower.