YS - thanks for those thoughts.
ISTR from my now long-gone PPL days that the key issues when determining length of T/O roll were air density and weight, with the former determined by altitude, humidity and temperature. I'm not sure to what extent they affect a modern commercial jet and of course I wasn't timing the roll with which I started this thread, so my report can only be subjective. It didn't seem to use as much acceleration as I'm accustomed to though so I assumed (it now turns out possibly wrongly) that this was in the plan (reduced thrust) rather than a result of low air density, resulting in lower engine thrust.
But it's interesting that take-offs are routinely performed with less than max thrust: I was taught that runway in front of you is way more valuable than runway behind you, so the quicker you take-off, the better. Maybe there's a difference between a 747 and a Cessna 152...