It really depends on how much damage was done. The "timing" on any engine is between the crank shaft at the bottom of the engine, which is attached to the pistons, and the camshaft which regulates the inlet and outlet valves in the cylinder. If the timing is "out" then the pistons come up and hit the valves as they are out of synch. It may only be the valves that are bent which means the cylinder head comes off, new valves fitted and thats it. Bigger problem if the valves have damaged the pistons, then they will be replaced also. £400 is a good price but watch out if they find the pistons damaged the price will jump. I would go with the repair but tell the garage to stop when the head is off to get a better idea of the true costs. Good luck.