Cable down to the shed

[Terry]

I think you'll find that this earthing problem is a complex one.

I would suggest that you post a request on the  uk.d-i-y Usenet news group where you will find lots of helpful advice from professional electricians.

I think it will be useful if you can also provide details of how the existing supply is earthed as, from memory of previous such requests, I believe that has a bearing on the answer.

There is some help in identifying that on their excellent Wiki:  http://wiki.diyfaq.org.uk/index.php/Earthing_Types

You may find other advice regarding your query if you investigate the Wiki in depth.