Belated thanks and further information
A somewhat belated thanks to those who answered my query.
I wanted to talk this through with my instructor, who provided his insight into the two options. He took the same line as Whopity - saying that regardless of whether it's NPPL or PPL, you still have to learn to fly to a safe standard. In his experience, this takes pretty much the same number of hours - only a very few exceptional pupils of his (1-2%) would have achieved this in the theoretical hours stated in the NPPL syllabus.
This suggests that for mere mortals like me the additional cost of PPL involves perhaps 2-3 additional hours of instruction on Ex 19 and radio nav. The General Skills Test for PPL is a slightly different format, but similar content and flying standard.
He also commented that learning in a Flexwing and then transferring to SEP may also end up taking many hours and overall cost just as much or more. (Bit late for me to take that option anyway, but nice to have that reconfirmed).
This is quite different from the impression I had formed earlier, that an NPPL might be a lower cost stepping stone towards PPL. He advised that unless there were medical reasons, going for PPL would be a better option. As mentioned by others in this thread, it also opens the door to further opportunities in the future.