However, the supplementary information also says:
The sleeper does have set down only restrictions in Devon/Cornwall.
For two reasons, I don't think this prevents the OP from doing what they have proposed.
Firstly, the "INFORMATION" panel merely explains the intended effect of "set down"/"pick up" restrictions for splitting in the context of typical splits involving single/return tickets. It clearly doesn't apply to season or other tickets where non-stop splits are permitted. The capitalised term "Call" is defined in the NRCoT so as to exclude stops with the above SD/PU restrictions; condition 14.3 says that the train "does not need to Call". Therefore, splitting at a restricted stop is no different to a non-stop split - in either case, it is permitted.
If one adopts a purposive interpretation of the NRCoT, the historic intention behind condition 14.3 was to allow people to extend their journey without being restricted as to calling patterns. Therefore it would be incompatible with the intended purpose of the clause if the fact that the train made a SD/PU stop somewhere - even if not a "Call" due to the SD/PU restrictions - prevented you from "extending" your journey.
Secondly, the start of the NRCoT reminds us that:
We have included a number of ‘information’ panels and/or footnotes to help you understand the meaning of certain Conditions. Please note that these panels and footnotes are for explanations only and do not form part of a Train Company’s contract with you.
Therefore the contractual position must be determined without reference to the "INFORMATION" panels; the panels cannot be used to try and infer additional restrictions or limitations. Moreover, even if there were deemed to be any ambiguity about the intended effect of the clause (which there clearly isn't, as above)