With the amount of rain we have endured recently, your problem isn't surprising!
When I had this problem (at similar depth) I was advised as follows -
Dig another hole as close by the pond excavation as possible (having regard for safety) but slightly deeper and pump out from there, preferably into a water course which you know won't just recirculate the water. It can be tedious - particularly if you have reached the level of the long term water table, but it simply isn' practical to 'block' the water ingress.
When (if) you get it dry, make sure you use a continous membrane below the base and blockwork, otherwise the water will find it's way in and make painting or fibreglassing impossible.
When I re-filled the hole I used rubble and stuff to create a low level soak away - I don't know if it works, but it seemed a good idea at the time!
Good luck
Posted on Jul 11, 2007, 10:31 AM from IP address 86.151.95.27