Thanks to all who gave me advice after the HV shook me up with her declaration that my dd was light for length and dropped from the 50th to the 9th percentile. I have since had her 9 month check (3 weeks after the hv episode) and met the loveliest hv ever. She said that dd was clearly fine, and that some hv dont really look at the baby (rather than the chart). I showed her the WHO chart and she seemed familiar with it.
Also, and most shockingly, she measured DD and she is on the 9 percentile for length (so in proportion) not the 50th as the previous hv made it. I am genuinely concerned that the previous hv may have deliberately measured her wrong (1.5cm out), as it was only at my request that she measured her at all (until then I was all "oh, I don't really care about those charts...)
So for anyone who has a bright, active, healthy bf baby who is dropping percentiles, find a hv who has some brains!