Jon, to the best of my knowledge that's not possible on CT.

My understanding (have never done it) is that it is possible to join two gpx files together using a text editor. Google is your friend in this case.

For purely visual purposes by placing each day in the same folder it is possible to see the whole route on one map

See discussion here:

https://cycle.travel/forum/thread/5210cycle.travel/forum/thread/5210

However, for future use, CT offers the reverse - a long route that can be broken down into separate days (and files) by using the "Overnight" feature. It is really a very useful feature for multi-day rides showing distance and elevation at a glance. 

Thanks, I realise now I should have done it as one route and then split it. I'm using the folder feature which is really handy.