I have done Mark but tend to quiver tip mainly. The head of chub has diminished alarmingly. Last spring/summer I was coming home form work and stopped at Bletchingdon road bridge. I had a loaf of bread and wanted to see how many chub were present above and below the bridge. A few years ago the water would have been boiling with fish as you chucked the bread in. But I never saw one. Not a single chub and that was one of the most depressing things I can remember about this once terrific chub river.
I sympathise with you Alex. I'm not to clued up on your beloved Cherwell but have experienced similar on a stretch of the Wey that I've fished for umpteen years.
During closed season work parties I always liked to take time out and stand on a footbridge that defined the lower boundary of the club's stretch and feed the chub. They were always there in May and June partly due to dog walkers feeding the resident ducks, and it was always a good way to spot a potential target fish.
They no longer seem to gather there in springtime for some reason which seems strange after 30 odd years of watching them.
It's no coincidence that that particular spot is a prime poaching area and I know they get targeted by some,... shall we say, unscrupulous types!
Their numbers have definitely declined but they can still be found when employing a roving approach with mashed bread and crust in winter.
Due to poor weir operation and the stupid EA flood defence removal of overhanging cover, their preferred habitat has disappeared. There is no signal cray problem on that stretch.
It certainly seems as though some rivers are experiencing good times with chub right now, while others are witnessing a decline for what ever reason.