Be interesting to know the source of Interpreter's £31 and £51 extra charge per one way pax. About 3-5 times what I'm expecting provided the project is financed by some pension funds which will accept a moderate payback period on a regulated asset.
Both those figures are roughly double the estimates that have been published by other sources, though the relative proportions seems about right given that Gatwick has around half Heathrow's annual pax.