Formula Required to Lookup and Match Multiple Criteria

Hi,

I've got a schedule of orders and subsequent invoices raised and would like to establish which invoices had original related orders.

I've attached an example of what i'm working with and included some additional narrative in comments boxes. I've attempted the first formula myself which does the job but i'm not sure it's the most efficient? i'd be grateful if someone could point out an alternative/better solution?

If you have any queries just give me a shout.

Thanks in advance,

Snook