I think someone asked this on Armynet, and think I covered it on here as it's somewhat confusing. Some say it's after you've completed Trg Gap or whatever basic you have to do.
I believe I moved onto Private without a trade qual after my first year, however a lad who joined about the same time, and did a trade, was still on a lower rate for sometime after me. Mine went up shortley after I queried it with uber admin bods (and PSAO in the bar - good place to do business), the other lad struggled for ages after and no one seemed to know what should have happened.
Now officially I do not hold a trade (due to driving quals required and lack of time, otherwise I'd be C2), but am one or two levels up on basic Pte. At one point in my life I went from Â£30.96 to Â£36.47 pd - but this point does not seem set for all people, like my colleague (joined at same time, had done more training) who had his increment about 7 months after mine. Maybe it's worked out on Bounty Quals, or on celestial shifts. I suspect that it's on completion of the basic training package (excluding trade training), or one years service and someone ticks a box on unicom/whatever.
As well as the above, we go up an extra level when we get a trade qualification (including driving qualifications as necessary).
Note: Your chain of command should record on your file when you finish your recruit training/ trade course, but you also need to check that they have done this. If the info needs to be added late, you should get your pay backdated so that you don't loose out financially.
Further Note: Pay also goes up slightly each April due to inflation