At last I have managed to visit my boat and check all the settings as suggested.
Firstly I have had the firmware checked by the local Raymarine
dealer to see if a new version was available and if that would cure the problem of the missing map display and the two screens not talking to each other.
At 6nm none of the Preveza area would display and it was a similiar story for other areas and other range settings.
My firmware was version 4.11. Which is the latest available. However, the technician performed a full reset on both the 80C+ and the 70C+ and, hey presto, I could get a proper display of the chart at any range!
I say could, because three days later it started to drop the charts
The problem he suggested was the new level of sophistication with the latest C-Maps. They just push older chart plotters to the limit. I will re-perform another system re-set when I next visit and see if I can get back the full display capabilities.
He couldn't help me with the 80C+ not sending waypoints and stored routes to the 70C+, but following the instructions of Noelex 77 worked and the two are working in harmony.
Anyone considering a system re-set should bear in mind that this needs to be followed by a re-calibration performed through the auto-pilot control and involves sailing a 360. I am still out of the water
until the 2nd June so this should be interesting as there seems to be a lot of button pressing to get to the right menus and I have already forgotton most of what I was told to do.
TIP: If you ever get a technician showing you how to perform a task film it on your iPhone
for later referral.