Have you tried to select the lines using the meatballs on the right side of the sub-layers? Are the lines masked using clipping mask at the layer level (done with the clipping mask icon in the layers panel)? You should be able to select them with the Direct Selection or Group Selection tools. Try unlocking all items Object>Unlock All, or Command+Option+2. Do you know how to work in isolation mode? If not, you may be in isolation mode for another object.
You can try press double click on that object. It will release the isolated group. Can be press twise or thrise.
Thanks for your suggestions but unfortunately it hasn't as yet solved the problem! everything is unlocked and none of the sub layers are locked etc.
When viewing the illustration in outline mode I can select each of the lines by using the direct select tool and clicking in the centre of the line, however if i want to click on the end point or grab all the lines in one go by dragging my select tool over all the end points it won't select anything. I just don't understand why it works perfectly fine in CS3 but not in CS4?!
Maybe because you started using CS4 without reading enough about the changes?
Masks in CS4 work differently because users like you and me were unhappy with the visibility of mask contents beyond the mask (but you probably knew that).
So now objects outside the mask aren't visible and don't get calculated when you align, transform or otherwise manipulate the mask+object.
One consequence is that you can't grab objects from beyond the mask in the traditional ways. It worked "perfectly" in CS3 but not in CS4 because the application changed.
Try entering outline mode and grabbing endpoints beyond the mask with your lasso tool.
Go into Isolation mode
Excellent alternative, WZ.
Thank you so much for this, what you've said has totally explained things and selecting the end points using the lasso tool works great!!