dave_perry,
Yes, that would count as one of your 2 attempts at the Aptitude Test. It's not limited to two just for pilots, but two for everybody. Even me as someone applying for Engineer only gets 2 attempts.
Fake Sealion,
From the sounds of things this is an AFCO interview or similar? In that case it will be very similar to the OASC interview: roughly split into 2 parts, the first all about the candidate, their past, interests, educational history, etc... the second about current affairs, RAF knowledge, etc.
So, for prep, the best thing to do is write down everything about yourself [especcially those bits you wrote down on the application forms] and know it well. Be able to say names/dates quickly.
As for the current affairs, watch tv news, read newspapers for a good 3 weeks beforehand. Go to the RAF website and get to know the equipment as well.