Found this when I logged into my Amazon Prime
It is available to claim until December 2021 but once a book is borrowed you have 30 days to use your voucher
- Offer expires at 11:59 p.m. (GMT) 31 December 2021. Borrow an eBook with Prime Reading within 30 days of signing-up to Amazon Prime to receive a £5 credit (“Offer”).
- Credit can be activated within 30 days after having borrowed your first eBook with Prime Reading. You can activate your credit on amazon.co.uk/prime.
- Once activated, the credit will be automatically applied to your next eligible purchase.
- Credit expires 30 days after it is activated.
- Credit applies to physical products sold on Amazon.co.uk by Amazon EU Sàrl or Amazon Media EU Sàrl . Products sold by third-party sellers or other Amazon entities will not qualify for this offer, even if "fulfilled by Amazon.co.uk" or "Prime Eligible".
- Credit does not apply to baby and infant formula, e-cigarettes, delivery and handling charges, gift wrapping, alcohol, Amazon Gift Cards, Warehouse Deals and digital products including Kindle ebooks, digital magazines, audiobooks (including Audible companions for your Kindle eBook purchase) or digital subscriptions such as Kindle Unlimited.
- This Offer is only be available to customers located in the United Kingdom or in the Republic of Ireland.
- Credit will not reduce the price of the items to which it is applied, though it will reduce the amount you pay. Accordingly, shipping charges and VAT will apply to the full value of any items that you purchase with the credit.
- Offer limited to one per customer and account.
- Offer may not be combined with other offers or promotional codes.
- The maximum benefit you may receive from this Offer is £5. Credit has no cash redemption value. Credit must be redeemed in a single order and will be allocated proportionally among all items in your order.
- This Offer is available to Prime members only.
- Amazon reserves the right to modify or cancel the Offer at any time.
- Offer is non-transferable and may not be resold.
- If any of the products or content related to this Offer are returned, your refund will equal the amount you paid for the product or content, subject to applicable refund policies. The credit will not be available for re-use.
- If you violate any of these terms, the Offer will be invalid.
- Amazon.co.uk Terms & Conditions do apply to the Offer.