does the voucher have to exchanged before the match or can i just use the voucher on the day?

You usually only have to redem vouchers before the match if they think it is going to sell out, you might be ok to just give it in on the gate

okay, thanks ill give the ticket office a ring just to check

I saw people at the ticket office today with junior vouchers and the woman at the office told them just to exchange them on the gate on the day.

Yeah, just hand them in on the gate, you'll be fine.