Good afternoon Michael and welcome back!
Thank you for reaching out to us.
We are sorry to hear about the 404 page creeping up on your site. Yikes!
I would love to help you with this topic.
As a first troubleshooting step, could you please provide us with your complete system information in a private reply using the instructions found in the following link?
https://theeventscalendar.com/knowledgebase/sharing-sys-info/
Secondly, could you please send me a copy of the latest version of your WordPress theme / child theme via a link to a .zip file link (stored Dropbox or Google Drive) so that I can run some tests on my end ? I recommend a private reply for that purpose.
Please ensure you are using the latest files as found on your actual website.
This way I will get access to any updates or customizations you might have made.
Finally, I am noticing some odd messages in the Google Chrome Developper tools’ console when viewing http://www.manchesterbikeshow.com/event/manchester-bike-show-2018/
You might want to take a look at that.
Hang in there!
Geoff B.