Just an update to tie everything together. I'll bullet point it, to make it easier to follow:
(1) Initial issue was only getting 13 gears, as per title. Bottom line is that in the end, this was a cable management issue, tied in with the fact I had the OEM axle plate incorrectly positioned. However, from what I learned over the course of these messages, I'd recommend the first port of call for anyone experiencing the same problem would be to remove the EX box, and use an 8 mm spanner on the nut that's normally inside the EX box to check that they can get 14 gears (13 clicks). Also, check at the same time that the twist shifter moves freely from 1 to 14. If the answer is "Yes" to both of these, use the 8 mm spanner to rotate the nut fully anti-clockwise into 14th gear. Rotate the twist shifter to 14th gear also, and then replace the EX box. (Of course you could do the opposite - rotate the nut fully clockwise and put the twist shifter in 1st gear). Especially if the EX box has been removed at some point recently, this to me is the obvious first thing to try.
(2) Thanks to eagle-eyed PH, I realised my axle plate was (a) in the wrong position, and (b) not secured by a nut running through the modular drop-out, to sit within the U-shaped notch. This was also causing my upper cable to be incorrectly positioned, jammed up against the chainstay. I think for most people, this won't be the issue, but always worth checking. I've explained how (I think) this happened below, with a "note to self and others".
(3) I discovered (again) that searching for solutions to these types of problems can be quite difficult, especially because not all Rohloff hubs are installed the same, depending on frame type, etc. In fact I have only found one photo on the whole of the interWeb that has a similar set-up to mine - the Shand YT video demonstrating how to remove a Rohloff-equipped wheel (albeit their wheel is QR and mine is thru-axle). Of course, I'm sure the answer is somewhere in the Rohloff manual, but it's one of those Catch 22 situations where if you don't know what you are looking for, you won't find it, even if it's there somewhere ...
Note to self and others: If you have a new bike that is equipped in a way you are not familiar with, take detailed photos of anything and everything. That way, if you ever take it apart, or wonder if something has become out of position or out of adjustment, or may be missing something, you can look at the photo. I *think* my axle plate issue arose when I removed the mudguards I had fitted. One of the bolts that secures the rear mudguard runs through the mudguard eyelet, through the modular drop-out, and out the other side, to - you guessed it - the U-shaped notch in the axle plate. I remember removing the mudguard, screwing all the bolts back into the frame/dropouts and then trying to re-install the wheel, but I couldn't get the EX box into position. I realised that it was because the torque arm couldn't rotate far enough anti-clockwise - the bolt described above was sticking out too far, preventing the axle plate from rotating any further - so I removed the bolt thinking it was long in order to go through the mudguard eyelet. What I didn't realise was that the axle plate must have moved (clockwise) while I had the wheel off, that I should have removed the bolt far enough to rotate the axle plate into its correct position, and re-installed the bolt. Lesson learned!
Thanks for all the replies, you really helped.
