Spot on Rob. Thanks.

That's exactly what I need and I'm very tempted to purchase it for the next time this happens. It seems to happen about once a year so I'll probably just buy it along with a spare cable so I'll be ready for it. This time though it'll have to be done the difficult way as I've already got my cable and will be doing it tonight.
It's easier than that.
Get a long tube with two bungs with holes in just big enough to feed the new cable through.
In one of the bungs you will need another hole that can be be bunged with another 5 mm bung
Thread the new cable through the one bung and into the tube.
The thread it though other bung and insert the bung into the end of the tube.
Your tube should be long enough so you now only have 10mm of inner poking through the second bung.
Pull it tight and secure it with an old fashioned brake cable bolt, the type where the bolt has a hole in it.
Now your new inner cable is stretched straight in the tube fill the tube with liquid nitrogen using a funnel and the 5mm hole.
Then bung the hole for a couple of minutes.
Now poor out the liquid nitrogen and remove the now more rigid inner cable which in its more rigid state should thread neatly down the bike frame.
Simple!