Hi All
The IGT charge is applied by all companies, including ScottishPower I am afraid. Its a standard charge which although it collected by your supplier it is passed onto the IGT.
Realistically the only one who has won here is the builder, they put out to tender for installing gas mains and meters to their new builds, Transco (who used to do all the UK) would quote say £1 million, but the other IGT's say one of them the Gas Transportation Company - GTC (one of the biggest) would quote £750,000 whose your builder going to choose ? yeah the IGT they are after saving money.
The IGT then install everything and connect their mains onto the Transco network, then your supplier has is charged by Transco for the transportation of your gas from their storage, via their mains, then your supplier is charged by the IGT for the gas coming into their mains pipes up to your house.
So I'm afraid realistically it ain the supplier but the builders who are winning here.