Jesper (and anyone else who is following this thread)
This morning, (hey, I was in early.. 06:00hrs, far too early really), I reloaded the 412 configuration from project 1, checked that I had comms with the 315 configured as in project 2. Comms were OK.
I got hold of a different 315 (6ES7 315-2FA03-0AB0 V2.1) and just configured that as a stand alone PLC, again networked via profibus, but nothing hanging off the network. Downloaded the configuration to the PLC plus a DB for writing to. Plugged the profibus cable in and away we went.. we have comms. Just to make sure that the 412 would talk to the 315 (old) I powered the 412 off then back on again, comms came straight back up. I then powered the 315 off and back on, same result, comms working all ok!
I still find the behavour of the 412 confusing, configured as in project 1, it talks to anything (well, at least two different 315's), but configured as in project 3, no joy!
So now I have one 412 and two 315's to play around with, the 'old' 315 I removed from a project that I am woking on at the moment, I will have it in my office till Friday, then I will have to put it back where I 'nicked' it from.
Paul