I've now worked out how to use the following formula:

=SUMPRODUCT(--(A1:A10="Jon"),--(B1:B10=B"),(C1:C10))
which gives me the summed value for jon & B's.

I have now realised that I have a volume column that shows how many of that
particular row there needs to be in the summed value.
e.g.
Name Category Value Volume
Jon A £20 2
Max A £40 1
Jon B £30 3
Mary A £35 2

Would i simply add this extra piece?

=SUMPRODUCT(--(A1:A10="Jon"),--(B1:B10=B"),(C1:C10),(D1:D10))

??