Hey Nigel,
Sorry you’re having some trouble here! If I understand your questions correctly, then it seems like the most straightforward way for you delete many old events in bulk is to head to the “Events” page in your wp-admin.
Once here, notice a tab in the top-right of the screen called “Screen Options”. If you click it, an option for how many events to display on the admin page will show. Bump that number up as high as you can, which depending on your site and server should be up to 100 or 150 events per-page (beyond that, and it will just load too slowly or could possibly even fail to load depending on your server).
So then you can just sort events by start date or something, and then “bulk select” a bunch of them and then just “Move to trash”.
This should be quite a time-saver. I hope it helps!
Cheers,
George