I'm pretty sure that if you catch sea trout in the sea then you don't need a permit and the bailiffs can't touch you but anywhere within the river then the fishing club/riparian owner has the right to all migratory/non migratory game fish and while they can't stop you taking sea fish like flounders in the tidal reaches they can stipulate out of bounds/non fishing banks providing the fishing owner also owns the land.
I did at one time know the law backwards and forwards but there's been so many changes over the last few years that I'm not 100% certain. Ask the bailiff if in doubt mate (be sure to get his name in case you meet another with a different opinion though)
