thank you for the feedback Christopher - I didn't make notes as I was in a bit of a rush and it wasn't straightforward (for me).
In the call with Ross, we checked that I'd followed the process on the page mentioned earlier and then ran the following, and also went into IAM & Admin, Org policies etc... (you can see the history below)
`
gam info domain
gam oauth create
gam info domain
gam config customer_id C02pt857l
gam config customer_id C02pt857l domain russellcourtarchive.org timezone local save
gam user administrator update serviceaccount
gam user administrator check serviceaccount
gam version`

apologies for the abysmal "notes". We had to repeat a few things, I don't know why, and we finished by upgrading to the latest version of GAM
Thank you very much Ross!