Hi Simon, sorry for the delay in getting back to you.
I have an iphone 5S. The main thing I want in a sat nav is to have live traffic updates and for the GPS to redirect me to another route before I run into the gridlocked area (not sure if GPS has this!). This would be more about local traffic (London) than on motorways.
Would downloading an app to my iphone do the same job? I think the Tom Tom 825 Live needs to use data from the smartphone anyway. You mentioned apps before so just wondered what your thoughts are on this option as a cheaper option?