I`ve just gone to open a completed invoice on here but there`s no details on it of what was originally invoiced for. Can i trace the original invoice

Welcome to the QuickBooks family, @geraintnewman.

I can get this figured out for you in no time. I recommend you start with using the audit log feature to trace any changes made to transactions from the users.

However, please note that you need admin access to perform the process. If you only have the normal user type for the account, I suggest reaching the Master Admin of the company to assist you.

You can access the audit log section through these steps:

1. Click the Gear icon in the upper-right corner and select Audit Log below Tools. 
2. Press the Filter drop-down.
3. Make the necessary changes.
4. Choose Apply.
5. Search for the invoice.
6. Hit View. 

Another way that you can try is by searching a transaction through the Magnifying Glass icon in the upper-right corner (see additional screenshot). 

I've included an article you can use to handle transactions, like how to find them through the account history: Find, review, and edit transactions in account registers.

If you have additional questions or concerns while working in QBO, leave a comment below. I’ll be happy to help you out.