How to fix my Baxi Bermuda PW5 Deluxe using a 401 Back boiler?

The problem is showing the following fault:- The pilot light stays on no problem but. After using the boiler for heating or hot water. The boiler cycles up no problem. When it cycles down there is a loud click then both the pilot and burner shut down instantly. The pilot can be relit and stays on but once again after a bolier cycle both shut down instantly.

Details for model are Baxi Bermuda PW5 Deluxe with a 401 Back boiler