It'll never happen. It's political suicide, regardless of how needed it is. Of course, capacity can be hugely increased without another runway - bin the curfew and it'll increase capacity more than adding the runway. Cite the lower noise of modern equipment versus the noise of types in use when the curfew was implemented, and restrict the curfew hours to the quietest types as an alternative to the runway and see what Londoners go for, but give them the choice of one or the other, not neither.