Does anyone know if its possible to "top up" AS qualifications?

[hazleweatherfieldgirldetective]

Just that, really. Is it possible to build on an existing AS qualification rather than having to restart the whole course from scratch?

I would check with the exam board to see if this is possible.
If the specification (syllabus and module content) has stayed the same then this is definitely possible but may be more tricky if it has changed.
For some subjects (Science) the specification changed in 2009.

If you know which exam board it was with you can check online as you can see the syllabus, but you would need to know the exam code qualification. Would you being doing it through a college, as some A2 have coursework which you can only do if you are with a college. For the sciences there is an alternative to lab work if you are not in a college. HTH