Leezygeezer.
There is no legal obligation to specifically carry out a legionella test. The nonsense banded around by estate and letting agents is designed purely to generate THEM income from mug landlords.
There IS However, under exsiting legislation which has been in place for over 20 years, an obligation and duty of care from a landlords perspective to ensure there is no possibility of legionella developing. If, for example, there is more than one shower head, the landlord must issues instructions to ensure the tenent knows how often to run teh unused to make sure its safe.
The legislation does not call for a plumber specifically you are correct. However, it calls for a competent person who is, much more importantly, someone who can assess an installation.
The ability to assess an installation means understand what a building is likely to have installed, to be able to recognise that having one thing installed almost always means somethng else is hidden away for example. The simple fact of teh matter is that it would be a rare individual who had the ability and knowledge to do that without being soemhow heavily involved in plumbing or building services. This, I suspect, is what you meant by your second post.