It happens after the loo is flushed when the systen is just filling or coming to the end of the flush.

Do you know if you have air chambers or hammer arestors?
Water hammer occurs when a fast acting valve such as a toilet or a single lever faucet closes. The sudden stop of the flow of water causes vibration in the pipes and the pipes bang against a house structural member.

A hammer arestor is a commercially available device as shown below. An air chamber is simply a vertical piece of pipe with the upper end capped or sealed. Both devices trap air which act as a cushion when the flow of water is abruptly stopped. Over time the air trapped in the chamber is absorbed by the water and must be replenished. These devices are usually but not always hidden in the walls.

Turn off the water to the house. Open all faucets or taps and drain the pipes. Wait a period for water held in the pipes by vacuum to drain. Close all faucets or taps. Turn on the water to the house. The air trapped in the pipes will move to the highest points, the air chambers, and cushion the sudden stop of water flow and the banging of pipes should cease..