NATS usually finds its new assistants (ATSAs) from within e.g. failed trainee controllers. So, unfortunately, it is very rare that NATS looks for new staff outside of the realm of the company. However, vacancies still do arise which cannot filled from within.
I would suggest the best course of action is to send your CV to the personnel manager at Manch, I believe it is then kept on file for six months and they will contact you if anything comes up. If you've heard nothing in six months then repeat the process.
Also be aware that NATS employs new ATSAs on a non permanant basis i.e. if there are going to be job losses, you're going and witout any redundancy payment! Unless, I understand, you can get promoted to a permanant position.
Hope this helps.