Just to reassure you, Garfield, I had a similar situation with dd2. Having fed the first two little porkers with no problems at all, this one took nearly 4 weeks to regain her birthweight (9lb 5oz), and has dropped to well below 25th centile.
The midwives were all very supportive, in a 'let's keep an eye on it but she looks fine to me' kind of way. I did get some humphing from the HV. I came onto MN to whinge, and got a resounding chorus of "Well, stop taking her to be weighed, then!" Which I duly did. I have no idea what she weighs now, but she looks fine to me, though she is on the dinky side, considering that petite is not really part of our family gene pool. Coincidentally or not, dd2 is the only one of my babies to have consistently slept through the night from 4 weeks. Other babies born at the same time who're still feeding two or three times a night look enormous in comparison.
My very no-nonsense GP told me that the thing to check with babies is the tops of their legs. If they have little chubby folds there, they're aren't going to be wasting away any time soon. Also, babies who aren't getting enough to eat are likely to be either miserable or very lethargic, ie. not bright, alert and contented.
Agree with other posters that the whole 'knowing how much they're getting' thing is a red-herring based on a bottle-feeding culture, and not helped by the ubiquitous GF, whose regime is actually very bottle-biased, tho' she never comes out and says as much.