At my Stage 1 yesterday everyone was wearing a suit. One or two of the ladies were dressed a little less formally, but only a little. I would assume similar dress would be appropriate for stage 2 - even if it is only computer tests there will still be NATS staff who are there to run the assessment and may well be taking note of things like how you dress. I may be wrong, but I certainly got the impression in stage 1 that we were being observed quite closely throughout.