After struggling with 2/3 of daughters bedroom floor with lots of swearing and the odd joint which still gapes a bit, I think I finally cracked it for the last third!
1) Dust out the slot in the already laid planks with a soft brush2) Lightly sand the half cylindrical mating surface on the next plank
3) Put plank in at 20 degrees up from flat and line up4) Waggle plank up and down whilst slotted in to bed in the mating surfaces until the plank will lie flat on the floor whilst fully clipped in (about 10 waggles)
5) Admire!