I want to turn off the water supply to my hot water cylinder, to repair a hot tap. There is a gate or wheel valve to carry out this operation but I can't turn it. There is a nut in the middle of the wheel. Do I have to loosen this before I can turn the actual wheel.
These valves are prone to jamming as they are used very occasionally. You could try loosening the gland nut, & squirt WD40 on the spindle. Also try tapping the top of the spindle lightly with a hammer. BTW, you don't have to loosen the nut in the middle of the wheel, this only secures it to the spindle.
Do you live in a calciferous (hard) waters part of Southern Britain? Proneness of gate valves to jam usually occurs in these areas because very small amounts of calcium get deposited in the valve over time. You can try putting a bit more force on it using a wretch but don't overdo it.
Definitely do not touch the gland nut.
You will find it a damn sight easier to ensure the immersion heater if fitted, is switched off, any other form of heating to the tank is also off. Shut of the mains water isolator coming in to the house. Turn on all the hot water taps til they run dry. Sorted.