Hi Fraser,
There are a couple of issues I see with your setup:
1. From your question it seems you are trying to set a limit of 1 (Quantity) per user role. But on your setup you are using General tab> Amount types > Sum option. I think you should change it to General tab> Amount types > Quantity.
2. “I have then gone to Users/Edit and selected the appropriate GBP amount and minimum product to 1.” If you are trying to set the limits by user role, you don’t need to edit anything in the Users page in WordPress. Instead, you should set the option from Order min / max amount > User roles > Amount by user roles > Enable then save the settings. After that you should be able to see various user roles you have for example, Vendor, Guest etc, on this page. You can then set the minimum limits here for each user role.
Please let me know if you continue having trouble with this. In that case I would request if possible, you can share your WordPress dashboard access and we can take a look for you. You can send your details to [email protected]