Urgent replacement or not?

Hi, any help would really be appreciated please? I've just had a plumber say that the small expansion vessel needs to be replaced before a service can be completed. This is an unvented hot water cylinder. Can the hot water still be heated/used in the meantime? And how long for? Thanks

Hi, any help would really be appreciated please? I've just had a plumber say that the small expansion vessel needs to be replaced before a service can be completed. This is an unvented hot water cylinder. Can the hot water still be heated/used in the meantime? And how long for? Thanks