So, the solution ...
1. I have removed the GP-150 from the system, and will send it off to Furuno for repair as suggested by Bill and Alex. I’m cruising so that could take some months to coordinate!
2. I have reconfigured the MFD12 chart plotter to output GPS, Time, Heading, Motion and Waypoint data on NMEA0183 port 2.
3. I have connected MFD port 2 output to both the Hydra and the ICOM 505. (Port 2 output was previously unused and Port 2 input is already connected to Hydra output, so that is a sensible connection).
4. Hydra and ICOM now both correctly display position and time.
5. Hydra now displays heading and waypoint information, and can correctly display wind direction etc.
6. When the GP150 comes back repaired I will slot it back into the system, wired to the MFD and Icom as before, but I intend to leave the MFD feeding the Hydra as I have it now.
Point 5 above solves the problem I posted about last year where Hydra would only display angles relative to the boat. It can now also display correct wind direction as well as the waypoint data. Happy to share the wiring and MFD configuration changes as required.