Here's the thing about toxic chemicals. You dump them, and for a while you don't hear anything about it. Then 30 years later someone has to pay 100 million pounds to clean up your mess.
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As Joel Bakan points out in his excellent book 'The Corporation' a number of companies are repeat offenders when it comes to environmental laws. The reason? The punishments aren't strong enough, if a company decides the fine for breaking the law is low enough they'll break the law and pay the fine.
My own, badly thought through solution would be to issue fines in the form of equity. So an offence like this might result in Monsanto forfeiting 1% of their stock to the UK government, to be given up proportionally by existing stakeholders. (So if I own half of Monsanto I give half of one percent to the UK government). The government would have an obligation to hold the stock in public trust for x number of years before being able to sell it again. Regular corporate offenders would risk nationalisation, and stockholders might finally start to exercise their duty to regulate the moral behaviour of a firm rather than wittering that all management has to do to be sure they are doing the right thing is maximise value.