Brilliant, thanks.
Took a bit of working out, but I downloaded it and done that. Problem is I can't seem to change the start point and it seems to be using that as the other point to split between.
I have just tried to do it for the first time and the following works in BaseCamp for a circular route. In summary, you need to split the old track at the new start point to create track fragments and then paste them back together in the right order.
Split the route at the new start/finish point to create route fragments comprising a track from the new start to the old finish and a track fron the old start to the new finish. To do this just right click on the track at the new start/finish and select "split track". This will create a fragment from the new start to the old finish and from the old start to the split point. Save them as numbered fragments so you know which one is which. BaseCamp will number them for you.
Now create a new track by combining the track fragments in the correct sequence. To do this, double click each track in turn to open its waypoints window then Ctrl A Ctrl C to copy all the waypoints. Open the waypoints window of the first track element and paste the second set of waypoints behind the way points in the new first track elements's waypoints window.
Useful tips:
If you get the odd extraneous point, delete the last way point before pasting the next block of way points in.
It you are unsure which waypoint is which, click on it and it will appear circled on the map, use the down arrow to follow the waypoints in sequence.