If the company for which you have vouchers goes bust, you don't have formal rights to get a refund - but you can ask. Use this letter to ask for your money back.
[Your address and contact number]
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’m unable to use my [voucher/gift card].
I would be grateful if you would issue a refund to the value of the voucher to my address above.