I have a water hammer problem when I flush my downstairs wc. could the problem be my ballcock system as the noise comes from there when i move the shut of valve?

Yes, that is usually the cause because the Ball Valve into the Toilet is opening and closing too quickly. Suggest you replace Ball Valve with a new one. Bill Pipe Dreams Plumbing Services Bournemouth

9 times out of 10 it will be the wc inlet valve

Most likely to be the float valve, either the diaphragm or the ball valve has a minute hole in one of them. Change both, easily done. Turn off the supply to the WC. Flush the WC. Remove top of cistern. Sponge out any water that is left in. Carefully undo the nut connecting the water supply to the bottom or side of float, making sure you retain the fibre washer. Remove the plastic nut connecting the float valve to the cistern and remove. Fit new float, tighten up new plastic washer connecting it to cistern, ensuring the black plastic washer is on the inside. Connect your water supply back up, don't forget that fibre washer. Make sure the ball valve is free in its operation, ie when you lift it up and down does it catch on the syphon or side of the cistern. If it does adjust slighty by undoing the plastic washer and turning so that it moves freely. If not turn the water back on, check for leaks from connection to cistern and supply to float (2 places). Problem solved, you could always get a plumber in, cost anything from £35 - £60