Excel Formulas - Only Add Up Certain Entries

I am trying to work out a formula, but im having some difficulty, below is what i am trying to do. I need to be able to calculate how much i have billed a client for each month

Sheet 1 - sales ledger

Date Client Amount
14/2 client 1 £100
15/2 client 2 £50
15/2 client 3 £300
16/2 client 1 £200
17/2 Client 3 £500
1/3 client 3 £100
5/3 client 1 £400
10/3 client 2 £500

and so

sheet 2 - Monthly totals for each client

Client Jan, Feb, Mar, Apr etc
Client 1 total for that month amount

etc....