Viridian is now Energia. If the company is buying back its own shares at the moment, they should be able to tell you how to redeem them if you email [email protected]
However not all companies are interested in buying back their own shares. You might need to use a share trading service to sell them on the open market. For a one-off sale, X-O might be your best bet:
https://www.x-o.co.uk/
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