Thanks guys --
Weaver, I'm going to go with pouring in some PVA glue onto the lump of lead already there (about 70 grams of it). I'd heard about Gorilla Glue and I've bought a bottle of their PVA type. Making up the u/c sponsons will determine where the nose gear bay goes, but it looks like it won't be far away from where the real A310 bay actually is, so until I get the hole for the bay cut out, I'll have to put up with the chunk rattling around for the moment.
Long time ago I used to use small lead shot, when my Dad had replaced a bunch of windows around the old house we lived in (in the UK), there were these ceramic pots inside the sash frame which were filled with it, these pots were counter-weights for the windows to make them easier to open and then keep them open. Didn't bring any of it with me when I emigrated to Canada though and I had pounds of it back then.