This is the place to discuss not just allotments but all general gardening problems and queries which don't fit into the specific categories below.
(formerly allotments and tips, hints and problems)
I've heard the bottles thing is because the sun reflecting on the water acts as the deterrent....
I remember a trial of different cat repellants a few years ago on Gardeners Question Time & they concluded the most efficient one was citrus peel soaked in eucalyptus oil. I think. It was definately an essential oil, the peel alone didn't work. Tea bags soaked in the oil were also good
I find having my own cat works well. Other cats don't come into her territory (well, only to try & steal her food) & she doesn't crap in her own territory either. I suppose this means she's going in someone else's garden, but it is impossible to control a cat's toilet habits.
Honestly - it is sold in our water garden centre - £8 a box - the idea is that the bigger the cat the little ones flee. It is in small larger than pea size pellets.