when filling in the work experience section, it it necessary to include ALL work experience we've ever had?
It’s not necessary to do so, but I would only exclude things if you think other work experience supersedes or duplicates the entries you won’t include.
Old entries that may seem less relevant, like part time jobs in retail during your college or university studies, can be really powerful in showing your ability to time manage. However you might want to avoid really short stints in such employment at the same time, especially if you have other similar entries that are for longer periods.
You ultimately get to choose what you include in an application, but I wouldn’t rush into not including jobs. It’s only if you really have lots and lots of jobs over a very long period of time that I think you should be selective.