I hope and pray that your building contractor has wired the premises correctly, ie: a wire from the outside grey box/capping where the BT underground feed is .... direct to that master socket ........ and any other extensions wired onwards from that master socket ??.
Alas, I've visited more than a few new build estates whereby
they have run the wire from the grey box/capping, to a central point in the house underneath the floor boards (or sheet boards as they are these days), and then 'teed out' all the sockets from this totally inaccessible point.
The only way around this, if indeed this is the case, is to re-run a new wire. Who needs that though, when all wiring is currently buried and out of sight !!
Fingers crossed for you.