Thank you again for your additional helpful suggestions. I have already bought a second battery as the manual indicates a full charge will only give just over an hour of recording. We will be cruising the Norwegian Fjords so do not want to miss anything we can't go back to the next day.
Have performed some recording tests today, inside and outside. Only 5 clips that total just under 6 minutes of video. Playback on the 42" TV is very good. Have put the clips through iMovie and exported to a USB drive, giving a file size of a bit over 900Mbytes. This also plays well on the TV.
The above test suggests that an hour of edited video at 1080/50p will occupy around 10Gbytes. Therefore,I will try your suggestion of using 50i HA format - although I assume interlaced video files may take longer to initially process in iMovie, as it constructs and stores complete frames from the interlaced input.
Thank you once again for taking the trouble to provide me with so many useful suggestions. My previous video experience has been with DV tape, with simple recording but more difficulty inputting to iMovie.