I wanted to avoid dismanteling it so I experimented and found the following approach using console mode seemed to work. I used a remote device, a PC running Linux in my case, to run console mode.
In menu M2, use the Info button and make a note of the IP address. Make sure you have a network connection between the RasPi and your remote device.
Start a putty session on the remote device. In PuTTY Configuration, enter your noted IP address in Host Name (or IP address) and click Open.
Login as: pi
You should see Portsdown DATV TX etc.
Select Shutdown and reboot options.
Select Exit menu to command prompt. You should get a Linux prompt like pi@raspberrypi:~$. Enter:
cd /boot - the prompt will change to /boot $
dir - this should list cmdline.txt as one of the files.
sudo nano cmdline.txt - opens a text editor. You should see a big long line of text with commands in it. Use the arrow keys to navigate towards the end of the line and just after rootwait type in ip=192.168.2.47 (substituting the IP address that you want to allocate instead of 192.168.2.47). Space it like the other commands. Enter ctrl-O to Write Out and save the amended file, then ctrl-X to exit the nano editor. Enter exit to quit the putty session.
When you reboot Portsdown, M2/Info should show the new IP address.
I don't know my way round the Portsdown system & I'm not a Linux expert, but I am sharing this in case it helps.
If you want to give the PI a fixed IP address then it's the IP address you want to give the PI. This command set's the address the PI uses on the network.
For example my LAN network has adresses 192.168.123.(1-254). My computer is 192.168.123.36 and the router (which sets the addresses if they are not fixed) is 192.168.123.254. One of my Raspberry PIs is at 192.168.123.38.
The LAN network address of a Pluto defaults to 192.168.2.1 and putting this on my network will not be seen by my computer unless I change it to something like 192.168.123.1
(Some LAN networks use addresses like 10.1.1.24 so if you have this then your LAN network address will need to be 10.1.1.(1 - 254))