Colin, I have always bought my A7 spare keys from sellers at the bigger Autojumbles, Beaulieu etc. They usually have hundreds of used keys. You should have a number stamped on the barrel where the key goes in. However, it might be worth trying this seller on ebay https://www.ebay.co.uk/itm/CLASSIC-CAR-KEYS-CUT-TO-YOUR-CODE-FA-FP-FS-ST-MRN-EJR-RL-CODES-ETC-ETC-/281221381038 Expect to pay about a fiver.
If for any reason you haven’t got a number it will be very difficult. Consider buying new lock and handle from Seven Workshop or other A7 parts seller.