HazelNuts39 and Turbine D, Thanks for the replies. As I understand it, from what you are saying, the three stages stay at pretty much the same ratio of RPMs to each other, but I'm still not sure whether the RPM of N1 changes significantly with altitude, all other things being equal?
since posting the original question, I've since found this thread:
http://www.pprune.org/tech-log/31248...rb211-rpm.html
This suggests that air density makes a difference to rpm, I'm curious by how much though. Does the fan (and the rest of the engine in proportion) not unload and increase rpm when at cruise altitude and speed to a significantly higher level compared to when the aircraft is static and near sea level?