Teacher who do exam marking

[AyeWhySwim]

I'm interested in becoming involved in marking for exam boards in addition to teaching but am a bit baffled by the range of boards and courses that people mark.
I teach English and would like to know about any experiences other teachers have - recommendations on what's good to do (or otherwise).
What else goes on through the year in addition to the main summer and autumn sessions?
Thanks!

The APs and unscannables come from literally anywhere bar those i mentioned, today I have done schools 20 miles away, in the south west and London .

Thanks Rosie. I suppose if it's all anonymised or pseudonymised then it doesn't matter really. I was thinking about where I studied (different country), where names are clearly written on exam sheets, nothing computerised and they have to be sent by post to different parts of the country to avoid any conflict of interest.

I have an English lang/Lit degree and am doing a masters, do you need a teaching qualification to mark the papers or can you be trained?

AQA expect you to have taught the subject for a minimum of two years.