The next morning only a mugs worth of water had been collected - it certainly could have been a lot worse. The best news (as leaks go) was yet to come though - it seems it only drips when the wind is coming from a certain direction. It continued to rain heavily over the next few days but no more water came through.
Saturday dawned dry and sunny so we got the ladder out to have a closer look. The roof felt all seemed to be securely in place, which didn't surprise us. If the problem had been water puddling then it would have been a much more consistent leak. The only thing we could see was a slight unevenness in the leading and its sealant. Joe tapped it back against the wall and we decided to leave it at that.