Johnd.uk,
Wow. From the looks of it, they've done one of three things with yours...
1. Upgraded the type of eMMC storage used to a much faster type.
2. Upgraded the system to now using M.2 SATA SSD storage instead of eMMC.
3. Already installed an SATA SSD pre-sale and that's what it's using. If you take the lid off you'll be able to an SSD already installed there if this is the case.
Whatever the case, with as fast as that's showing, unless you need the extra storage for your system drive itself I wouldn't bother using the new SSD for your system drive. You're unlikely to see any difference.
That's really surprising, and pretty darn fast. It looks like you got a really good one. If you'll notice the snapshots of the speed tests in my review, what you have now is practically as fast as what I have AFTER installing the SATA SSD, which is REALLY good. Again, you got a good one. Now I'm jealous.
Regarding your question, yes, you can use the method referenced by the admin for cloning the drive, if you still want to do so. Personally, with as fast as that "from the factory", onboard storage apparently is, I would just use the add on 240GB SSD for extra storage and stick with the onboard storage for your system drive.
If #3 is the case, however, and there is already an SSD there, it again comes down to whether you want to use a 240GB SSD instead of a 120GB SSD for your system drive, but cloning to the 240GB in this case may be a little more tricky, as the onboard storage may not have Windows on it at all (it still may, though), which would mean you'd have to clone from an already added on SSD to one you're going to replace it with, which would likely require an SATA to USB adapter, so you could clone from added on 120GB SSD to the 240GB SSD you're going to replace it with, but that is temporarily connected to the unit via that USB adapter to allow the cloning. Either that or install Windows onto the onboard storage, then clone it from that to the newly installed 240GB SSD. I don't know whether this is the case, but looking under the lid will tell you.
If there is already a 120GB SSD under the lid, you also have another option, which is what I would probably do, and that is getting a USB enclosure for the new 240GB SSD and using it for storage via USB. You can get a decent USB 3.0 SSD enclosure for less than $10 at Amazon. With this, you could connect it when you want to save files, or even have it connected all the time to use it as a constant backup drive. Again, though, looking under the lid will let you know whether you even need to make a choice like this.
Don't worry, it sounds a lot more complicated than it really is, and if you need advice or help I'll be happy to lend it. Others may as well. Just check under that lid to see whether there is already an SSD there.
For reference, eMMC is a type of soldered-on flash memory (storage) that is used in things like tablets, cell phones and less expensive mini PCs. It's similar to what's used in SD cards, like those that are used with cameras and such, but is permanently attached to the system board.