Use a spirit level or a piece of string on two pegs to give you the right line for the bricks to be reinstalled. Remove the bricks and reset on a bed of sand using your level. Bang back in place with a rubber hammer. (fairly quick job!)
Marnie: The above answer is exactly my solution too!
You can use a straight board or a level across the bricks when you reset them. Sweep sand in between the joints to lock them in.
Hi Marnie, these videos should give you some instruction
Cement helps keep the sand from washing away in heavy rains
I agree that pulling them out and releveling is the best method. Some of your base probably has washed away.