Hi @Pete1
Welcome to the forums! 
If I’ve understood you correctly, I think you’re looking for the bulk entry tool? This would however only be for a single client/supplier and a single nominal code. If you have the Power User Subscription, it is a bit more flexible.
Take a look at this tool, let me know what you think. You can find it under Account Settings > Import Data
Edit
Details on the Power User version can be found here