As per the above post.
When NATS changed the policy from making everyone do an aerodrome course before proceeding onto an approach or area radar course, there was no longer trainee ATCO's leaving the NATS system after maybe failing one of the radar courses or failing to validate at unit.
In the past non NATS units could be sure on receiving a number of CV's per annum from these trainees looking for employment, there was almost a surplus.
With the adoption of the the common core basic course, then streaming to tower for London units, tower/approach for others or area the people who tripped up in the system often left with nothing to offer other units & probably left air traffic all together.
Often these pre-basic course trainees who did leave with a tower ticket took anything they could get, got further training, but often a few years down the line moved to either a perceived bigger/better unit, or closer to thier home town. This in turn meant there was always movement within the market place.
Non-NATS units then became much more proactive at training from within thier own ranks i.e. ATSA's who prodiminatly were local based and therefore much more likely to remain loyal to the unit. With all the extra money required to fund the extra training, the hope of getting this needs no explanation.
This in turn means more stagnation within the market place as people are less likely to move unless the T&C's are very attractive with relocation allowances, rent paid for X amount of mouths to help settle & find property to buy, significant salary increase etc etc but in the present climate it's harder for units to justify these perks when being told to save money.
Viscous circle that may take some time to clear.