It was an assumption I made (first mistake) based on them being similar.
The 15-ax200na, the one I bought stated the 256gb M.2, nothing else.
The 15-ax205na, same price, same spec except it lists as a 128gb M.2 and a 1tb HDD
The caddy space is in the laptop and there is also an unused header on the board right where one would expect to see the SATA connector for you to slot a 2nd SSD in.
Bitterly dissapointed about it, I could always send back the Samsung 500gb 2.5" i've bought and replace the onboard M.2 with a 512gb. But still feel a bit cheated out of the 2nd option.