Hi Sam,
I'm in the same situation. Did my stage 1 last week, and had the personality questionnaire in my inbox before I got home! But also have not heard anything else. The lady conducting the tests last week said there had been some trouble recently with the online system so probably give it time.
I have been told though that receiving the questionnaire is a pretty sure response that stage one has gone well... how true this is still remains to be seen!
Don't worry about it, give it the full 15 working days as they ask and then if there is still nothing heard drop them a phone call.
Hope this helps, and good luck!