Just to complete the story so far. SSH appears to be set up correctly. I get this:
C:\Development\Wireshark>ssh -p 29418 USERNAME@xxxxxxxxxxxxxxxxxx
Enter passphrase for key '/home/my_userid/.ssh/id_rsa':
**** Welcome to Gerrit Code Review ****
Hi Paul Offord, you have successfully connected over SSH.
Unfortunately, interactive shells are disabled.
To clone a hosted Git repository, use:
git clone ssh://USERNAME@xxxxxxxxxxxxxxxxxx:29418/REPOSITORY_NAME.git
Connection to code.wireshark.org closed.
If I then try git review I get this:
C:\Development\Wireshark>git review
Problem running 'git remote update origin'
Fetching origin
Permission denied (publickey).
fatal: Could not read from remote repository.
Please make sure you have the correct access rights
and the repository exists.
error: Could not fetch origin
There definitely seems to be a problem with the linkage between git and SSH.