Maybe instead of this whole circus (because believe me that is what this is going to turn into), AirServices could take a big leaf out of Norwegian ATC's book. That is, realise they are short of controllers, realise they have homegrown controllers overseas in large numbers who have gained valuable experience, and would be willing to come home for the right package but certainly aren't that desperate to grovel at their old employers feet for a job back.
Norway, contacted each of the controllers individually, and asked them what location they would like to return to, and when they would be wanting to return. Voila, they all have start dates back in their workplace of choice, at the start time of their choice, at a pay rate that recognizes their experience level. They are happy and the Norwegian ATC provider is happy because they now know what staffing levels they can plan around. And guess what no overseas recruiting junkets required by anyone.