Agree with Gonzo.
The pool of ATCO's has changed considerably in the last decade because of the reasons Gonzo cited.
NATS has recruited allot of experienced non NATS controllers recently and yes NATS has lost controllers to ANS.
The recent pay reviews at ANS and Birmingham demonstrate that there is a need for ANSP's to start reacting to the market and review their terms and conditions (that doesn't always mean more money).
An airport mentioned earlier has lost 6 ATCO's in the last year with ATCO's attracted to better pensions, annual pay reviews and overall improved conditions.
Fundamentally there aren't enough experienced controllers to fill the vacancies, and when an ANSP doe's advertise, it can take 3 months plus for the recruitment process (depending on the HR department).
If you find a suitable candidate, they usually have a 3 month notice period. The training (which depending on the unit) can take anything from a few months to over a year depending on the availability of OJTI's who need to ensure their own competency hours are maintained.
ANSP's need to retain their experience, and that starts by looking after the staff they already have.