Thanks guys, so in summary
There is some problem waiting for the associated groundworks to stabilise (not the extension itself).
It isn't long enough for the heavies to use for full long range flights ie west coast USA or far east but ok for middle east and east coast USA.
Finally as said most pilots prefer more rather than less, understandably. I see on the virgin thread that with the reduced braking capability Gatwick was very tight on length for it's landing.
Good to see airline use is on the up
Re undulations I imagine most runways slope one end to other and many have undulations,no doubt there are specifications covering this for new runways.