Graham mate, this thread is about Hertfordshire not London. Tongue in cheek comment maybe, but poaching is not the best way to go is it. We have a big problem with poachers on the colne. Both clubs that I belong to on the colne are very thorough in their bailliffing, both day and night. One has a waiting list to join, the other has a huge membership.
As for not catering for guests, I have just offered polo a guest ticket,which will cost me a tenner,and cakey has tipped Broxbourne . The river Gade is all free fishing or day ticket, the chess is a trout stream. The Lea has some free bits and day ticket bits.
If you want to fish in London the Thames is free fishing from Windsor (Berkshire) to the estuary,as is the G.U. canal,and the large reservoirs do day tickets.. And you will see quite a few anglers even in the heart of London,most of whom are friendly and helpful.