Something that I think more people would do with garden ponds is to arrange a feed to them from the rainwater gutters - and then an overfolow to a drain or soakaway. This makes for a much healthier pond - and very few people collect anything like the full ammount of rain that falls on their roofs each year.
I have 2 x 400 gallon rainwater tanks - One heavy downpour can fill them - but the overflow goes to my pond.
I'm sure that with Monty's resources he could arrange something like that for the GW pond