So I've checked out the Tgi3d plugin.
Sort of does what i require. the difference being that i was hoping for incremental selection using the arrow keys, i know realise how i should have explained it sorry.
so it would go
1 select a line
2 select a direction
3 use arrow keys to select how much of that segmented line you need
the tgi3d plugin does all the line in one go up to a border or a junction, so there might be use in my way