After some rooting around on the Internet (pun intended?), I discovered that it is actually quite easy to set up.
For this you will need a small USB stick, I used an old tiny 4GB thumb drive.
Ensure it's partitioned and formatted to ext2. If you're using Windows then EaseUS Partition Master can help, there's a free version for home use. Otherwise the Gnome Partition Editor, otherwise known as GParted, will work.
Just pop the USB drive into the back of the router and go to the "USB application" page in the UI, it's one of the options under "General" on the left hand index.
At the bottom of that page is the "Download Master" which you can then choose to install onto your USB.
What this does is into install a package manager, along with various packages for the Download Master, but this gives you the ability to install other packages!
Next, in the "Advanced Settings" -> "Adminstration" section, about half way down the page in the "Miscellaneous" section is the "Enable Telnet" option. Set this to "Yes" so you can log into the router via telnet.
Telnet to the router and log in with your admin account.
Execute the following:
ipkg install net-snmp
app_set_enabled.sh net-snmp yes
the config file, should you need to change it, is /opt/etc/snmpd.conf
I found that after changing the file, to modify the community string for example, I had to execute S70net-snmp.1 in the /opt directory which, if you look at its contents, shuts down any snmpd and starts it again.