Is it possible to buy a £5 topup voucher and then redeem it to buy the £5 goodybag which is for unlimited texts?
No sorry, you can't. The minimum amount is a £10 voucher. Although from the £10 voucher you can buy 2 £5 goodybags so one is live and the other will be queued and will take over when your live one expires.
No sorry you cant.