If you dead head the first lot of large flowers, (if you can reach, depending on how large you buddleia is) but be careful not to remove the two smaller flowers at the base of the flower, these usually grow larger than if they are attached to a dead flower. And the plant tends to have a new lease of life and produces more new flowers. I have done this other years and it has worked well, but this year I was on holiday, everything went to seed and I was left with were a few pathetic small round flowers instead of large conicular ones.
I dont dead head the second lot of flowers as the Tits love the seeds in the winter and I think insects tend to shelter in them.
I don't actually cut my buddleias back until the first week of March though, like Paul says 1-2m as the new flowers will grow on that years new growth. This is the Davidii species I am refering to. I am no expert and my garden has been trial and error, but this works for me
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