Special consideration should be given to the earthing and protection arrangements for certain installations where reliance on the connection of the consumer’s protective conductor with the distributor’s combined neutral and protective conductor could result in more significant risks. For example, installations where it may prove difficult to attach and maintain all the necessary equipotential bonding connections (e.g. farms or building sites), installations at certain wet environments (e.g. swimming pools and petrol filling stations) and certain installations outside the equipotential zone of buildings (e.g. certain types of street furniture). Further advice may be sought from the Health and Safety Executive or the Institution of Electrical Engineers.
The above is taken from the ESQCR-2002 Section/regulation 9(4)
When it was the good old fashioned the Electricity Supply Regulations 1988 that actually had a section regarding exporting/extending the equipotential zone and you did in theory have to contact the DNO
Now though the ESQCR-2002 make the distributors maintain a much safer and more reliable network and so the dreaded lost neutral, though technically can still happen, is less likely.
So for an extending the equipotential zone out to a shed/garage no you would not have to call the DNO now. In fact if you did call them I would be amazed if
a) you would get to talk to anyone who had a clue what you were talking about
b) If you found someone that did, they would say ask an electrician