A simple way of going about it, is to input you're Advanced Subsidiary with the modules and the grades you achieved last year, then input the same Advanced Subsidiary again, but then only put the module that you're retaking, with the grade as pending and the date jan 2011 (if that's when you're retaking).
So, for example, if your subject is Ancient History, and you need to retake you're Roman History module, then input Ancient History Advanced Subsidiary, with all the modules and your grades from last year (including the Roman History module from last year), then input another Ancient History Advanced Subsidiary, with only the Roman History Module, with the grade set to pending and the date set at jan 2011.
Does this make sense to you? That's how they did it at my Sixth Form. However this may not be acceptable at yours. You can always ask your Higher Education/Universities co-ordinator at your school or college for more help. You'll have another check before Christmas though, with your Personal Statement completed.
Hope this helps, and good luck with the rest of your UCAS application!
EDIT: Okay, not a problem, it works the same though. Just impute the A Level with your previous grades for units, then input the same A Level again, but just the module(s) you're retaking, with them set to pending and the date as 2011.