The saga continues...
Thank you for all your replies so far. There is obviously a huge variation between airlines and how they meet (or not!) the departure requirements.
A few points I'd like to make. Generally we anticipate all jet a/c to accelerate to 250kts after departure, not withstanding those lovely British 146....., and A340s! The speed limit on departure is just that - a maximum (not to be exceeded like the one on the M25...!) and there's no restriction on flying slower. The problem was that the Towers use 'Speed Tables' to determine departure separations, and the A340 is officially in the same League as B744 etc. Even taking the sprightly VAA boys doing 250 kts, a B744 accelerates faster and will catch you unless we're careful!
Trying to second guess an A340's speed on departure only tends to result in an embarrassing situation so most controllers I know just wait and see what you do! I'm glad that the boys (and girls) at Heathrow have got the idea now and use 'common sense' departure separations behind A340s.
The 340-6's do perform (in ATC terms only I hasten to add!) much better than the 'Shorter, Slower, Less Far' variants! And I will quite happily wait until after I retire (32 years away...) for the A380.
If My OpenFly is correct:
>> ..... the dreaded 380! It has not been designed to go up, down or sideways, but go very fast on the ground.
May I suggest that it's best suited to following the new, freshly widened M25 round to Dover then using the cliffs to effect a take-off! No conflict with Gatwick traffic apart from passengers using the M23...
Regarding the A330, I'm sad to say it doesn't perform anywhere near as well as the B777, but again I guess this comes back to the business of accountants. Boeing like to provide a bit more welly! The A330 is far more efficient, and shares climb performance as well as it's wings with it's elder brother.