Unison and systemd timer problem

I've got a problem whereby if I run my bash script myself unison works as expected, when systemd runs it via a service on a timer then it does not work as expected. I'm logging the output of the unison executable and it comes back with nothing. What could be happening here?

The script in question is this...

    #!/bin/bash

    echo "`date -u` `unison "USB Work" -batch`" >> /home/tom/timer.log

...which when run from the commandline works perfectly. The file is /usr/local/bin/unison.sync.usb-work.

My service /etc/systemd/system/unison-timer-usb-work.service looks like this...

    [Unit]
    Description=Unison timer - sync work to USB

    [Service]
    Type=simple
    ExecStart=/usr/local/bin/unison.sync.usb-work

...the service is run every 10 minutes successfully by the following timer (etc/systemd/system/unison-timer-usb-work.timer)...

    [Unit]
    Description=Runs Unison USB Work sync script every 10 mins

    [Timer]
    # Time to wait after booting before we run first time
    OnBootSec=1min
    # Time between running each consecutive time
    OnUnitActiveSec=10min
    Unit=unison-timer-usb-work.service

    [Install]
    WantedBy=multi-user.target

All of this appears to be set up correctly, systemctl list-timers shows it as active...

    NEXT                          LEFT       LAST                          PASSED    UNIT                         ACTIVATES
    n/a                           n/a        lun 2015-08-03 08:50:25 CEST  8h ago    ureadahead-stop.timer        ureadahead-stop.service
    lun 2015-08-03 17:00:40 CEST  9min left  lun 2015-08-03 16:50:40 CEST  30s ago   unison-timer-usb-work.timer  unison-timer-usb-work.service

...and the /home/tom/timer.log file gets new blank entries with a timestamp every 10 minutes.

Why does the script work from the terminal (without sudo and with sudo) and not from the timer?!

Thanks in advance for any help,

Tom.