Re: SL1987
Sounds to me like you've far more experience of the industry than most, so I really wouldn't worry about that
To be honest I'm not sure NATS are really looking for much in this area beyond a little background interest anyway - I've no experience of the aviation industry except as a passenger and this didn't hold me back in any way.
With respect to the diagramming test, I think what Juffled is getting at is that you don't need to work every example out in its entirety. As the answers are multiple choice I generally found (particularly towards the beginning of the test) that once I'd done the first few operations this narrowed the selection of possible answers down enough that I already knew the answer. (Caution - it is worth checking none of the final operators re-arrange the earlier boxes though, or this could catch you out big time).
Time is tight on the diagramming.. this was the only one I didn't manage to finish. But if you get your head down and don't let it fluster you then you'll do fine.
Best of luck
Pat