It is not at the moment legally essential to get a certificated legionella test done, but it IS legally essential to do periodic legionella assessments.
You can do it yourself (I believe there is a testing kit that can be bought for about £30) but obviously it’s more competently done by someone who has taken and passed the ‘legionella testing course’.
A proper letting agency can arrange this for you, but the better letting agent will have a staff member who has trained to do the tests and assessments and so can arrange it in-house and obviously save the costs of outsourcing it to a specialist firm.
For in-depth info on the matter, check out the relevant sections of the HSE website.
http://www.hse.gov.uk/legionnaires/legionella-landlords-responsibilities.htm