Brian, thanks for the quick reply.

So a faulty TV/Video Module will disable a MK1 nav unit. Did I understand you correctly?

Yes, I think I will go straight to a MK4, then I can throw away the TV/Vid module (assuming I don't want TV) and remove the old Trimble module and plug the GPS antenna straight into the MK4. I prefer to upgrade in stages, where possible. So I would leave the old 4:3 monitor in place and replace that last of all. I know I need to get a 'conversion harness' to connect the MK4 nav into the I-Bus. Again, am I correct in this?

Maybe later I can change the radio module to a BM54 so I get TMC and other stuff, like MP3 support.