If the company for which you have vouchers goes bust, you don't have any formal rights to get a refund. But you can still ask for one.
Use this Which? template letter to help you ask the administrators for a refund.
Making a claim to administrators to refund vouchers
Dear Sir/ Madam,
Reference: [Voucher number and retailer name]
Please find enclosed a [gift card/voucher] to the value of [£xx]
It was bought from a [name of store] on or before [insert date].
The [name of store] has now gone into administration so I am unable to use my [voucher/ gift card].
I would be grateful if you would issue a refund at the value of the voucher to my address above.