This list displays payments starting from the oldest year first at the top, but the most recent month before earlier months! Consequently, the entry for November 2018 (the most recent payment) is on the line below January 2017!
The sequence runs: Dec 2017, Nov 2017, ..., Mar 2017, Feb 2017, Jan 2017, Nov 2018, Oct 2018, Sept 2018, etc.
This is just unnatural! (And is surely a bug?!)
My preferred order would be simply to have the latest transaction at the top of the list, and then the others in strict reverse chronological order. This is fairly common practice and is surely easier to implement than whatever GiffGaff is doing at the moment? Would also be easier for people with many pages of account detail (mine is 5 pages long).
A second irritation on that page is that after opening a 'View Details' link for a particular payment, browser-back has lost your place in the list so you have to go and find it again. I discovered that I could open the details in a new tab (using RMB menu), so I could compare the content more easily, but this could be made more obvious.
Just tried to repeat the behaviour using the Opera browser, which does not have the Ghostery anti-tracking add-on installed, and that page is behaving correctly (despite any default anti-tracking settings).
I have seen some strange effects on other websites, due to Ghostery's rather comprehensive behaviour.
However, despite disabling Ghostery, and CanvasBlocker as well (the other add-on) the page still doesn't work correctly in Firefox.
In both cases, Opera (57.0.3098.102) and Firefox (64.0) are latest versions.
Here's the list as seen in the Opera browser, displaying correctly: