Star - my dd2 did similar (but unfortunately I had been at baby clinic evety month so it was documnted)
she was nearly 8lb when born, but at 12 months old was still only 16lb.
she dropped off the charts completely by 4 months old or so, and straggled along - always gaining, but very slowly.
it triggered all sorts of FTT investigations which were (imo) largely unnecessary - dd2 was eating like a horse, developing well, lively and interactive. plenty of wet nappies etc etc.
she was tiny right up until the last 18 months or so, when she has crept into the small side of normal - major growth suprts on her part, but she is still smaller than all her peers.
she was ebf too (I fed her until 16 month due to family dietary issues)
I pre-warned the hv this time around, that ds might well do the same, and that I wouldn't necessarily be worried - she surprisingly agreed
(in a 'if it happens, we'll look at the child, not the chart' kind of way).
what is your gut feel? you might want to hold off on major allergens (especially gluten/dairy) - I did for dd2, because of dd1, iyswim. I know you don't have the gf/cf issue, but it could be something to consider.