you will need these if you are renewing or are taking out the dhw heat exchanger . the old ones will be hard and flattened off. clean the recesses that hold them before you put the new ones in. you can use a small mirror to make sure they are seated correctly.
Bought this diaphragm to fit in my Gloworm Jaguar 28 boiler.Fitted perfectly and easy to fit. Excellent item all round.Saved a fortune on boiler engineer fees.