I'm getting really sad responding to my own posting......
But another thought just popped into my head (I get about one every hour these days if I'm lucky).
It seems to me that the single runway option is easy. You get down to your briefed fuel for diversion (company minimum figure or more based on your own good airmanship), then off you go. No ifs, no buts. Just like the good old days.
The difficulty comes with the two runway option, LHR for example. Provided wx and approach aid requirements are met, you can eat into your diversion fuel (and the company will expect you to.) As soon as you do this you've burnt your bridges. Now something unexpected happens to delay things - and the puckering starts, through no fault of your own, or lack of judgement. It's just the way it works.
Only way to give yourself a buffer is to always carry enough fuel for your expected delay, plus.
Ramble...ramble....pilot's stream of consciousness.........
Dick, you hit the nail on the head - in the real world, therein lies the problem.