You should be able to get the jaws of medium sized adjustable spanner onto that at a slight angle unless you've already rounded off the brass. Then give it a sharp tap with a pin hammer or similar light hammer and it should go.
The wee horseshoe or C clip is to hold the threaded shaft of the valve into the valve body.
Thankyou. Our local plumber came with a spanner with a dog leg in it. It was very tight though.
Ceramic valve with cheap rubber washers. You could see how much it had worn down compared to the new one, plus feel how much tighter the movement of the tap was with the new one in.