I do believe it is necessary for us to draw the considerable distinction between, for example, departing JFK around a weekday evening time when a 30 to 45 minute taxy time is to be expected and the rather different case of being caught out by some unforeseen and unexpected event between engine start and lining up for take off.
Wizofoz: "States quite clearly in our FOM that it can be used anytime after the refueler dis-connects, so that includes APU burn BEFORE dispatch" - APU burn, (400kg/hr?), is not normally the problem though is it? It is all four burning and turning after start and before take off and I, for one, would not want to be using my contingency fuel whilst still on the ground.