Editing an existing node to add additional cck locations removes some previous cck location data

This is a slightly odd problem, and took a little while to track it down, but I'm using cck location on a node type set to unlimited values and also using the cck3 multigroup module.

If I add a new node and add e.g. 3 cck locations all is well, they display and show in the edit form if I edit the node.

However, if I go back an edit a node with 3 cck locations in it, and add one more, then only the first and most recently added (4th) locations remain in the node, telephone and fax numbers stay for the 2nd and 3rd cck locations, but the addresses vanish.

I can't see any similar issues reported for this, has anyone else had the same problem, and know what the solution could be?

Many thanks,
James