Does anyone know where I can find very short term inland waterway insurance for inflatable craft?

I signed up for a paddle on the Avon at Evesham this Saturday with a walking group I am in.
Unfortunately, no-one realised in that group each craft needs a licence to be on the river (albeit just £2 a day, I discovered) and I discovered this morning that £3M worth of insurance is required even for a wee plastic inflatable dinghy! The cheapest cover that amount is £30 per year and I cannot find any shorter-term insurance anywhere. As I purchased a second vessel so that my brother could join me I would prefer not to fork out £64 for just three hours on the water. Does anyone know of such insurance that covers less than a year?