I’ve never had one and my mate the sparky says I don’t need one!

If you are connecting a 15A RV/caravan/tool to a 10A outlet by illegally modifying an extension lead, then you are breaking the law as this contravenes standards governing mains voltage equipment. This may jeopardise insurance claims for damage caused by using an illegal connection method.

Illegally modified 15A leads are referred to as “death-leads” for a reason.