it is possible that you got your supply from your neighbours who got it from the mains, which would explain why you went off when the main was off, unfortunately you have come into their awareness now, so they will try and do whatever is in their power to make you pay. I still think that the citizens advice people may be able to help, at least to clarify your status under the covenant, a lot may depend on the exact wording on that, if it only states that water will be supplied then you might be liable to be charged, whereas if it says that it will be supplied free of charge, that will mean something else.
worth getting it checked out, either that or try the money saving expert forums, there will be people on there who may have more expertise in these things.
Grendel