Hello there, @davidbaines.
You'll want to go to the Sales menu and send an email to all your customers from there. Let me show how.
In your QuickBooks Online (QBO) account:
Here's an article you can read for more details: Email or print multiple invoices and sales forms.
I'm also adding this article that you can read in case you want to customise your sales form: Add, customise, or remove logos on sales forms.
Our door is always open every-time you have other questions. Getting you back in working order is our top priority. Keep safe and stay healthy.