I am not sure if this is a UK company or not. https://www.stretchcvboot.com/cv-boots/

This site has a picture of a Cone to stretch the boots.

https://www.walmart.com/ip/UK-NEW-CV-Boot-Fitting-Installer-Stretch-Mount-Cone-Tool-Driveshaft-Universal/608841636

There is likely youtube videos on stretching the boots over the axle.

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84 300D, 82 Volvo 244Gl Diesel