Excel 2007 : Formula Query: How To Apportion An Invoice Over A Period

Afternoon all,

I'm trying to create an automated prepayment schedule but i'm struggling with the apportionment forumla (to be honest I don't know where to start).
Attached is an example of the schedule in use at the moment. What i'm looking to create is a document where I input the relevant info in columns A to F and then the formula will apportion the invoice value appropriately across the relevant period. For example, in row 2 the document value is £3,600 and relates to a 12 month period (Apr 10 - Mar 11) therefore in April (col G) a month's worth is effectively incurred and 11/12's is carried forward to future periods hence the figure of £3,300 in cell G2 (£3,600 x 11/12).

Thanks in advance,

Snook