What do the electricity, gas and water companies require by way of
trench depth, separation of ducts, marking etc. for extending and moving
services to a fresh location?

I have found on-line suppliers of black duct, yellow pierced duct and
marking tapes but little further information.

The run is about 15m from existing supplies which are laid at around 18"
deep and about 12" separation with no tape.
--
Tim Lamb