A couple of points Shed and Bagso.
1. I don't think it has been or should be an absolute principle that all surface access works should be funded by the airport (or more correctly the air travellers). Two reasons. First, projects may produce benefits for non-air travellers. Second, there may be benefits to the wider economy rather than to the air travel system. So I don't think it's the principle, it's the numbers in the business case and their credibility which are important.
2. I don't think Willie Walsh can be regarded as a wholly objective player. BA could lose and UKplc could win. Dominant incumbents rarely welcome threats to their market position. If every airline CEO says 'Not at that price' then that would be different.
3. Nevertheless it is stonkingly expensive and maybe the 1800 metre R3 with no M25 tunnelling works which was being thought about in 2003 needs to be revisited at least as a Phase one. Such a long construction period with no payback is part of the problem.
I hope the Another Runway at Heathrow thread can be revived because there is a lot of mileage in this yet and this stuff would be better there.