cut a peice about 200mm longer than the hole in the joist.
put it in and hold it against the existing joist and screw it in place with batten screws (best thing since the electric drill).
- nail the boards to that.
- if you want to keep the nails all in a streight line cut a peice the same size as the gap and put it in and screw it in the gap. now you have a streight joist.
hoope this helps.