I had the same problem but it turns out it is only calculating to the end of this tax year (ie April) so no wonder we are getting virtually nothing!
Have you rang them yet? If you ring they will tell you how much you'll get. Give them an estimate of what your P60 for this tax year will say though, I made the mistake of telling them my new salary which meant they sent me the form saying I owed them when really the salary increase was only for the last 2 months.
horseshoe - when you say substantial increase, do you mean over £2000 per year? Anything under that figure and they should disregard and still work on the previous years income.
There are only approximately 2 weeks left in this tax year. £25 over 2 weeks is £12.50 per week, you were getting £11.50 per week. With the increase in benefits they have promised in the budget, it sounds to me as if you are going to carry on getting approximately the same amount.