Thanks Bob, I think we're getting there (although I'm not sure where 'there' is)!
The old fish tank is proving to be excellent for testing and I managed to sneak away for a few hours over the hols to cast the last ballast weights. For the record, here's a pic:-
I'ts her bottom with the main keel removed showing the entry to the ballast tank. The lead wedges pass through the hole and fit the hull shape. 1.2 kg in total!
Len