On Friday, 9 December 2016 10:55:33 UTC, Tim Lamb wrote:
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?

They usually have the info buried deep within their website, often in the 'new connections' section.

Western Power Distribution say

We require a trench which is 530mm deep and 300mm wide along its whole length. You can only dig trenches on your own land.
The trench bottom should be level and free from any sharp stones or objects which may damage the service cable.
Our cables are normally laid directly into the trench so need to be reinstated with 75mm of stone dust. In some cases ducting can be laid in advance of the new supply, and this ducting can be obtained from your local WPD office.

Draw cords should be incorporated and the ends marked for ease of location. Trench depths in pavement or private land shall be 530mm (min. cover over cable 450mm) and in roadway shall be 600mm (min. cover 520mm). Where cables are not protected in duct they must be blinded with 75mm of crushed stone dust and approved yellow €œelectric cable€� marker tape laid above.

Without charge to WPD the Customer/Developer will, where agreed by prior
arrangement, install suitable 38mm internal diameter WPD approved tubing
from the service position to the footpath or verge etc. as shown on the plan.