kb, I'd totally agree with "look at the baby, not the centile chart".
Dd2 was above the 50th centile when born, and now, at 26 weeks (totally bf until starting weaning last week) is just above the 2nd. She is, however, lively, happy, alert and producing plenty of wet nappies.
she is also fairly chubby (see pics in profile). She just doesn't seem to weigh very much.
My hv was concerned when she saw what weight dd2 was, until she took a look at dd2 herself. What she saw was a perfectly happy baby. And she relaxed (well, a little - she is now encouraging me to add cheese and cream to everything dd2 eats, to fatten her up a bit - she really does not need fattening up, she's just one of life's smaller babies).
I did have the odd moment of worry, but then I realised that dd1 is on 98th centile, and no-one ever told me that she was worryingly large. They just looked at the overall picture and realised she is happy. I do the same for dd2.