The way the PAYG costs stack up seems to be as follows (using current service costs from here
https://www.honda.co.uk/cars/owners/3-years-plus.html):
1 x Honda 12+ service = £290
1 x Honda 12 service = £180
2 x MOTs @£56 = £112
Total = £582
Which is pretty near the same as the Service Plan extension cost of £589.
BUT the PAYG prices are likely to increase over the next two years, plus if I stick with the main dealer I save two years worth of AA/RAC/etc subscription.
Jocko: yes, it would exclude any work arising
Derkie54: you're right about an independent probably being cheaper, but I kind of trust the local main dealer (odd thing to say, I know!) and as Downsizer says it'd cost me more on the breakdown cover. Difficult to get costs for an independent, but as a comparison it looks like Halfords would charge around £532 for the same deal using today's prices, which is cheaper but that advantage would be eliminated by the breakdown cover.
So it sounds like it's worth going for, the only downside would be loss of interest on that capital (but that's peanuts anyway).