Oi Gonzo!
TAD is correct! As Unit Training Officer, I introduced assessments for prospective Heathrow Tower controllers around 1992. Various NATS and non NATS controllers applied, but unfortunately the majority were turned down. Some made it to the OJT stage, failed, and subsequently validated elsewhere. The notable success we had was Paul T.....g, but assessments did not yield a huge number of successful candidates, possibly because the opportunity was not widely publicised by management at the time.