It would be foolish indeed to treat ARRSE as the definitive complete source of advice in these matters. I don't- it just happens to be a useful forum in which to drop a question in and see what the responses are. Like any forum where you don't know who the people replying are, you must take replies with a grain of salt. However, I like to think with a reasonable ammount of common sense applied that ARRSE can be a very useful supplement to all the other forms of advice and help out there. For one thing you can ask your daft question knowing that if you get shot down then at the end of the day- you've only made a tit of yourself on the internet, and lets face it you wouldn't be the first .
6 months ahead roughly. On the JIs I received (mid-December for May '11) I had to ring back the OCAC and re-confirm (again) the PCCBC and CC I was booked onto, do a new medical form, get my doctor to do a new medical form, fill in another ethnicity sheet and I've still got to finish my Security Vetting questionaire. Its a ball ache.
If you're on the September one they'll probably drop through your door in March/April.
Edited to add: you have to ring back OCAC within 28 days otherwise "they can't guarantee your place". Quote.