We are going to have an extension built on the side of our bungalow. It will start forward of our current side/main door and the meter is just back from the door. We are hoping to get the meter moved so that it will be at the front of the extension. Do we need to contact the supplier or the company we pay(British Gas)?
Neither. You contact the Regional Electricity Company for the area in which you reside (the ones whose name appears on the vans in the local area). These people always did retain the right to maintain the local cabling in the regional area. And they also retained the right to deal with meter installations.
E.G. in southern England it is Southern Electricity (or S&S as they now call themselves).
The electricity company will charge quite a lot for this work but you have no choice but to use them. Be aware that you need to have your own electrician on site as well as they will have to connect the supply after the work has been done. The elec company will not move the meter unless your electrician is present to do this.