I appreciate your quick follow up and thank you @natty88 however your answer does look very much like a template answer not specific to my mobile device platform/operating system. It also indicates that my question hasn't been read and the details of my platform noted before this answer was posted. The answer contains information that I've already seen and for anyone to find fairly easily using a Google search.
I have read your question through and responded to each of your points within it as follows:
"is the device unlocked or on the 02 network"
My (therobyouknow) response: The device is unlocked. It was not bought from a mobile retailer nor part of a contract.
"have you entered the giffgaff settings as shown here SETTINGS . . . . . . . these settings will fully explain how to set up your internet an mms on your device they include settings for. . Iphones ..tablets ..android..windows . .blackberry . .nokia ect . . check all 6 pages of settings
My (therobyouknow) response:
Yes I've seen that settings page. None of the entries specifically mention Windows nor Ubuntu. I could understand perhaps that Ubuntu is not mentioned as it is not that common at the moment for a mobile device but would expect Windows as this is a more mainstream mobile platform.
If I'm going to attempt a solution specifically based on the information on that page then My only way forward using the information is to attempt to guess my settings using a mixture of those on that page which at a guess might be similar to my setup.
"or if you have an android phone you may want to try here for settings which would help
". . IF RUNNING JELLYBEAN.. http://bit.ly/ggjellybean
"you can also get the giffgaff app from your phone's app store e.g. Google play for android
My (therobyouknow) response: The title of my post mentions Ubuntu and Windows not Android. Your reply is further evidence that you haven't really read my question.
"Or Manual settings could you try this guide via your mobile data settings an select add new apn or edit a prestored one EG 02 then enter theseSmiley Happy Connection Name: giffgaff
Proxy: Not set
Port: Not Set
Password: [leave blank]
Server: Not set
MMSC: http://mmsc.media messaging.co.uk:8 002
MMS proxy: 22.214.171.124
MMS port: 8080
Authentication Type:"PAP or CHAP"
APN type: default,supl,mms
APN protocol: IPv4
APN roaming protocol: IPv4
MVNO type: IMSI
MVNO value: 23410x also remember to click save once you have entered these setting an restart phone
My (therobyouknow) response:
This is perhaps the most useful part of your answer. However, several of the above fields aren't shown in the Ubuntu Network manager.
So unfortunately your answer hasn't helped, mainly because it doesn't refer to a solution for the specific platform that my problem is about: Ubuntu Linux 18.04 LTS 64bit.
So, going forward, apart from my guessing settings, the only other option is for me to research further and/or wait and see if anyone responds to my question here: https://askubuntu.com/questions/1079689/mobile-broadband-sim-doesnt-work-on-ubuntu-18-04-but-does-on-windows-10-pro-64b
I'll be sure to post here with success if I find it.