If your electric shower is quite old (10+ years?), or you live in an area with particularly hard water, it is likely there's sufficient limescale build up inside the electric shower that it will no longer work as expected and will require replacing.
With the assumption being every electric shower we've seen (besides Aqualiser) have been in zone 1 of the bathroom, you would want a qualified electrician undertaking the work, 1) To ensure the safety of the shower where you're bringing mains voltage in close proximity to water and 2) so they can sign off their work and notify building control.
