If like for like you have no obligation to supply any certification. I though would always supply a MEIWC simply because it looks more professional, it only takes a few minutes and it is a reference point for a customer if they require further work in the future
If though the existing circuit is not RCD protected and the new shower manufacturers instructions ask for one, then you will need to fit an RCD and then supply a EIC, SoI and SoTR as you are changing that circuits protection characteristics.
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