Ahhh that's great thank you
I was doing it completely the wrong way, thinking the ordinary categories module would allow selection!
For those also doing this, go to modules, make a new module, and like Indresh says, set it to be ordinary category, but then filter for a product category you want.
Then in SP Page Builder, add a row, add in Joomla module block, and select the name of the module you just made
Beware the name you gave the module just above filter will also appear so no need to give title.