Scarabeetle: my ds was 8lb 15oz at birth, lost over a pund initially and took 7 weeks to regain birthweight and thereafter gained weight very slowly. He dropped from the 91st centile to eventually following the growth curves from just underneath the centiles.
He was happy, healthy and alert throughout. If I hadn't been weighing him, I wouldn't even have realised.
Fortunately, I had qualified HCPs (the breastfeeding counsellor midwives at the maternity hospital) to support me who never once suggested formula. What they did suggest was supplementing with EBM, initially after each feed and then, for a short time, every alternate feed. Interestingly - although I was fortunate enough to be able to express loads (not everyone can - which is not a relfection on how much milk they are producing) - so that I could see that he was getting loads of milk - it made sweet FA difference to his growth curve which stayed on exactly the same trajectory.
By your logic, I should have given him formula so that he could have stayed on his centile 
I was referred to the consultant paediatrician, just for a double-check. he was more interested in what my father was doing (who used to be a radiologist at the same children's hospital) than in the manifestly happy, healthy and alert baby in front of him (7 weeks by this point, under the growth curves). He told me to "stop the faff of expressing, go on doing what I was doing, enjoy the baby and that eventually he would move back up the chart - that he was taking time to find 'his' curve". And guess what, he did.
I later learnt that ds was a claissic case of "catch down" growth: born "above" his natural weight (thnaks to my efficient placenta/good health and possible even borderline Gestational Diabetes) and therefore "needed" that period of flat/minimal growth until he could get onto "his " centile. Ultimately he crpet back up to the 50th and I successfully fed himfor 13 months. :)
splatt: what my BFC midwives did recommend in the early days, when he was a bit too keen to sleep through was to wake him 2 hourly (( have to admit I did 3
), then 3 hourly (= 4 in reality) and at 4 hourly (= 5)