Hi Kevster,
It's been a long time since I was recruited by NATS, so perhaps someone of a younger vintage will reply. I think that apart from interviews, they now do fairly standard verbal and numerical reasoning tests and also spatial awareness. This used to be based on a series of diagrams, asking you to determine which "blip" was moving and where conflicts were likely to occur. I would have thought that now this is carried out on a PC?
As a general strategy arrange yourself some visits to as many ATC Units as you can and ask for brochures. Ask questions, display an interest in aviation, visit the NATS website, familiarise yourself with the issues around privatisation, Mode S transponders, developments in Oceanic airspace (RVSM, datalinks, use of satellites to display aircraft position etc), New En Route Centre, New Scottish Centre etc.