Mill Barn is £15 for father and son ticket, I was in exactly the same position in that I thought it was expensive BUT... I soon realised that the price stops all the annoying kids and other idiots especially when the schools are out. I joined for £85 which I think is really good value considering the quality of the fishing.
The reservoir holds some nice carp to nearly 30lb I've had several good days with multiple doubles & low 20s. There is also a members only lake 2and 3 other waters.
Someone has also commented on the 10lb line rule and barbless hooks. If you can't land a 20 on a barbless & 10lb line you are doing something wrong!
If you want cheap fishing go to Priory Park for a fiver but be prepared for the kids and chavs.
