Bob is pretty much spot on here - the 'worst case scenario' is not all that unlikely either. If you were rated "Consistent with peer group" in both years you would get a 2% pay rise each time, leaving you with a salary of £29,651 after 2 years.
As bob says, if you joined just after the promotion date (september) you'd remain on this salary for almost another year as well. If you were rated "below peer group" you'd still be on the starting salary of £28,500
Best case scenario: Those rated "significantly above peer group" would get a 10% rise at the end of the first year, taking them to £31,350. They would then be a candidate for early promotion - I'm not sure what the current C1 salary range is but I'm sure someone can enlighten us. Bear in mind this happens only to the top 5% though.
You also get the sign-on bonus - 6k at the end of your first month and 4k at the end of your 12th month.
At the moment, if you add on the bonus it is still a pretty good starting salary. However, if you don't get promoted the 3rd year salary looks pretty poor!
Finally, I feel I should dispel the myth that this is a "grad scheme" - after initial training you are chucked into your first project and that's about it - apart from a nice trip to chicago it's unlikely you'll get any more formal training in your first couple of years. There's no defined "scheme" as such.
Didn't mean to write so much - hope this helps!
PS - if anyone at analyst / C1 level in any of the other big firms is reading this, how do your salaries compare? I would be very keen to find out whether the current low morale at accenture is justified!