a little advice on tapwater in the UK. i live just outside warrington and the tapwater here isnt too bad but i dont use it. if you want to know what your tapwater contains you can contact United Utilities on 08457 462200 if you give them your post code and a little info they will send you a copy of the drinking water register summary.
when you read it it is pritty much self explanatry.
as has allready been said copper is a trace element in NSW
but you are looking at at about 0.000254 ppm
which there is no way you will get a test kit that will go this low.
also i would say if you test for copper with the test kits available in the UK and get any reading, you will never beable to get it low enouth to put inverts in the tank. Once there is copper in the tank it will leach out of the rock and gravel/sand for years.
sorry im not being very posative but here is a little info on water quality that may help you.