Slightly scary electrical leakage

After a recent heavy rain storm , I touched the damp outside wall of the garage and got a mild shock. The garage is detached from the house. I immediately switched off the house main circuit breaker but this had no effect. I then switched off the mains outside the property which stopped the problem. From this I deduced that the leakage is between the outside circuit breaker and the house db board. Is there a way of establishing exactly where the problem is, how bad it is and how critical it is to get fixed? We use a 230V 50hz single phase system here.