Why they needed the NI Numbers
is already implied in the article - as payroll records are tied to NI Numbers, they needed them to deduct membership fees.
My wife got her letter a couple of weeks ago; it is full of weasel words explaining that they believe that, to the best of their current knowledge, no personal fraud appears to have been attempted, as far as they can tell, given al the facts that they are currently aware of....
The excuse is ful of about as many holes as their security - as far as I am aware, based on the fact that I am currently aware of, excluding those that I may become aware of in the fulness of time or which I may have inadvatently been aware of at some point in the past but have subsequently forgotten. E&OE