I'm hoping for some advice/instructions
Yesterday, the session didn't record on to the SD card, not sure why. I tested it at home a few times and it was fine.

I seem to remember a while ago, some saying sessions are recorded in the GT31 and you can upload sessions direct from the gps using the USB cable. Alternatively, I might be able to copy the saved gps data onto the SD card.

Can retrieve the session or has it gone fo4 good?

thanks in advance.

From the manual:
How To: Download and Upload Data

Basic Operation of Downloading or Uploading via USB cable:
Make sure the USB driver has been successfully installed on your host.
Connect the USB data
Power on the unit, go to NAVILINK page, press in the
TS to enter NAVILINK mode
Execute the utility, NAVILINKII.exe on your host
You will see the screen on the right displayed.
In [Navi.Data] command page, you can select the items and
Click the command button to start upload or download.
When data transfer has been completed, press the
Power/ESC button to reset the unit.
.....
.....

To Download Logger Data(SBP) in GT-31
Step 1: Select "COM Port"
Step 2: Change default folder if needed.
Step 3: Select "Log.Data" command page. Click
[Read] to start downloading logger data.
The downloaded data is saved as SBP file and
can be converted to plt,gpx,kml,nmea files.
Click [Convert] to start SBP file conversion.

NAVILINK File Naming
The file names are composed of device serial number and proper file extension. The file extensions
include:
wpt - Oziexplorer Waypoint
rte - Oziexplorer Route
plt - Oziexplorer Track
plt.nmea - Track in NMEA format
spd - history speeds file
sbp - binary packet data file (for DATA LOGGER)
sbn - binary data file (for MEMORY CARD)
txt - NMEA text file (for MEMORY CARD)
set - device settings file
gpx - GPS exchange format
kml - Google Earth compatible format

Brilliant, thanks

First thing I would do is try to save internal log to card.

Thanks Andrew, just this and it worked.