Can I insert a header for an estimate to describe the project etc

Nice to have you here in the Community space, @jacko2.

As of now, the option to insert a header for an estimate is not possible in QuickBooks Online (QBO). As a way around, you can enter additional information about the project in the Message displayed on estimate field. See the image below for reference. 

You can also check out this article that provides the complete information on how to personlise your invoice and estimate in QBO: Customise invoices, estimates, and sales receipts in QuickBooks Online.

Additionally, once your customer accepts the estimate, simply convert the estimate to an invoice. This way, you'll be able to invoice them easily without entering the info again.

Please let me know if there’s anything else I can help you with your QBO account. I’m more than happy to help you out again anytime.