I would draw your attention to section 161 of the Water Resources Act 1991 (a section based on legislation first enacted in 1974) which gives the National Rivers Authority important powers. Under this provision, the NRA is entitled to carry out works, not only in order to remove polluting matter and restore waters to their prior state where it appears (to the NRA) that such matter is or has been present in any controlled waters, but also in order to prevent such matter from entering controlled waters where it appears likely (to the NRA) that it will do so.
Most importantly, the NRA is given power to recover the expenses reasonably incurred in carrying out the works referred to above from any person who caused (or knowingly permitted) the matter in question to be present.
Section 161, however, does have its problems, and is usually only used by the NRA as a threat in negotiations.
Significantly, the NRA has no formal power to force a person responsible into carrying out the works. The NRA must therefore carry out the works itself before it is entitled to recover its reasonable expenses, and there is no formal procedure whereby it can establish legal responsibility before doing so in order to ensure that it will be reimbursed.
While it is to be hoped that the Government makes good these shortcomings in the current legislation, it must surely already be the case that there is in place a system which gives the authorities power to make the polluter pay.