Bagso
To answer your question about how long it will take to pay back the investment in R3 you need to understand how airport charges are set at Heathrow. These are regulated by CAA, and are set at a level which seeks to achieve payback (including a 'reasonable' rate of return) over the economic life of the assets.
Heathrow's policy on asset lives is on p24 of
http://www.heathrowairport.com/stati...ember-2013.pdf
The short answer to your question is that payback will be achieved over the (weighted) average asset life, no more no less. This is currently 23 years, but we might expect it to be a bit longer for the new assets being constructed as a result of R3.
The long payback is not necessarily a bad thing - many investors in infrastructure look for reliable returns over a very long period. In this way they can match returns to liabilities, especially for investors such as pension funds and sovereign wealth funds. There is a seemingly bottomless pit of funds looking for reliable long term returns, especially as Government bonds no longer offer much of a return.