Once filthy waters are now full of healthy trout and grayling. Although the infamously dirty River Thames has not yet seen water quality good enough for decent fly fishing, others within London have.

This has been a quiet revolution over the last 30 years, and I have gradually come to realise that in Britain’s towns and cities, there is probably now more water with trout and grayling living happily in it, than without,” Mr Pike says. The great exception is the Thames in London, which is still polluted by sewage overflowing from the Victorian sewerage system, whenever there is significant rainfall. “While the Thames is an awful lot cleaner than it was a few decades ago, it still has major problems in that whenever more than 2mm of rain falls in west London, vast quantities of raw sewage get dumped into the river by the antiquated sewage system,” Mr Pike says.