Ah, I tried adding a new simple product, that showed up as in stock so I think I figured it out. The issue was my products were set as 'downloadable', but I hadn't uploaded in the 'downloadable information' and so it showed as out of stock. I uploaded a file there and the product's now showing in stock.
Thanks for the help guys, the prodding towards simple/configurable products helped me get there in the end. Thanks!
For downloadable products, they should be created as 'virtual product'. Also when specifying the product details, at the bottom, ensure that the 'is this a downloadable product' check box is selected.
I had the same issue with Magento 2.3. In my case the solution was to manually set each of the simple products to out of stock, save, then change them back to on stock.