This is a map that specifically shows the National Cycle Network routes. It is a large scale map, and shows every road, other that streets in towns. It's therefore suitable for navigating around Cornwall properly, whether you want to follow the NCN, or go on to ordinary routes.
I do not understand Caroline Wilkinson's review. She is criticising it for doing what it is supposed to do. It doesn't have a map of East Cornwall on the back. What it does have is town centre street map boxes so that you can follow the route through the technical sections of town without getting lost, a basic function of a map. Many people who cycle through Cornwall may have started in Plymouth, which is in Devon, so the map makers have handily included a map of the city centre, again, so you don't get lost.
The map clearly shows the NCN route numbers, which corresponds to the signs on the road itself. Why criticise that? "I wish to complain that map stopped me from getting lost". There's no pleasing some people.
My only criticism is that the map is printed on ordinary glossy paper. This makes it quite expensive at £5. Maybe a waterproof paper option would be better, after all, it rains a lot here in Cornwall.
Other then that, an excellent map, and just the right size for your pocket or backpack.