Hi Mike,
There should be a way of either turning the handle by hand, or by using a stop key, which is basically a long bar with a handle and a 'crutch' type socket to accept the fitting on the stop tap.
With regards to the water company charging you the earth, you will be pleased to know that water regs 1999 stipulate that it is the responsibility of the water undertaker (wessex water in your case) to maintain the water supply from the main through the communication pipe to the stop tap. It is your responsibility from the stop tap through the supply pipe and into your home. So basically, you can turn away at the tap, and if it breaks, its the responsibility of wessex water to come and sort it out
What have you got to lose?
Chase Plumbing Academy