Ok, so let's tease this out.
If the ODCA have told you that the terms and conditions must be printed on the voucher and there is no mention of the 'use in one go' condition, then that seems pretty clear to me-how you use the voucher is up to you.
Unless the buyer's attention was drawn to the fact that it could only be used in one go by the retailer at the time of sale, I don't really see how the retailer can impose such a condition now.
In my experience, it seems that it is accepted practice to let holders of gift vouchers to spend them as they wish, i.e. if you spend less than the value, you got another gift voucher for the remaining amount. I'd iamgine it is hard for a retailer to argue that this is a burden on them.
I would go back to the shop and throw the ODCA and the Sale of Goods and Supply of Services Act at them and see what happens. Also, don't let the ODCA off the hook, they are funded by taxpayers money (i.e. you and me), and so have a duty to provide you with a service for your money.
If all else fails, could you sell the gift voucher to someone who may be able to use it as the retailer is demanding, or can you save it for a time when you are having a party etc. and require pastries for many?
Let us know how you get on-there have been a number of interesting gift voucher queries recently and it would be nice to add to our key posts.