I have just bought a coopers English bitter kit and the man in the shop recommended just adding 1kg of normal sugar but the kit says 500g of DME which is right?
DME is much better. Sugar will leave it with less body and more bitterness.
With kits, most people use 1kg Beer Kit Enhancer, it's 50/50 brewing sugar (basically normal sugar but better for brewing) and DME, so it boosts the ABV without affecting the bitterness or body.