Perhaps best to phone them and double check that you have done it right?
If you started work in September, then your child care costs for the last financial year would only be those you expected to pay in 2011/12, so for the months of September, October, November, December, January, February & March.
Using carrielou2007's example figures, then of £100 in term-time and £170 in holidays, and using rough weeks:
September-April
Autumn term (first half) was 7 weeks - £700
Autumn half-term £170
Autumn term (second half) was 7 weeks -£700
Christmas holidays (2 weeks) £340
Spring Term (first half) 7 weeks - £700
Half-term £170
Spring Term (second Half) 6 weeks £600
Easter holidays 2nd-13th April, so only 4 days count for this year (170/5*4) £136
Total those: £3516
Number of weeks: 31 weeks 4 days or 31.8 weeks.
Cost per week: £110.57
September-September
Autumn term (first half) was 7 weeks - £700
Autumn half-term £170
Autumn term (second half) was 7 weeks -£700
Christmas holidays (2 weeks) £340
Spring Term (first half) 7 weeks - £700
Half-term £170
Spring Term (second Half) 6 weeks £600
Easter holidays 2 weeks £340
Summer term (first half) £700
half-term £170
Summer term (second Half) 6 weeks £600
Summer holidays (6 weeks) £1020
Total those: £6210
Number of weeks 52
Cost per week £119.42
In other words, you'd be best to check with them as to how you should have declared a part year, because using those figures above, you are claiming your child care is costing you (say) £110.57 per week, because the summer holidays push up the average, but actually, there are no summer holidays between September and April, which is the period you've been eligible for help with childcare costs.