Oh and also... I've seen some stupidly low prices on (2nd hand) Sigma 300 2.8's. Not my #1 choice, but you might want to take a look around.
My first thought was the 300mm f/2.8's for under £1200!!, should take a 1.4x really well. By f/5.6 with the 1.4x should be razor sharp.
Or if your not quite as worried about the AF performance then the Tokina 300mm f/2.8 is a stunningly sharp lens. I remember the auto-focus tracked really well, but took longer than the 400 f/5.6 to 'grip' and wasn't as silent. Made a slight buzzy noise. On the Tokina we tried the 2x TC and at f/5.6 it was sharp and by f/7.1 it was sharp as a 70-200mm at 135mm. Tamron also amke a 300mm, don't know how that performs.
The Sigma's can be bought about £1200 used and the Tokina's about £900 used, you get 300mmm, 420mm and 600mm. All with AF and the 300mm/420mm options are going to be superb in lower light.