I think I have answered (roughly speaking) my own question by doing a search on this site on an old thread
So if anyone's interested.....year 6 and 7 service cost - approx (based on previous model)
I've just taken out a Honda "Tailored Servicing" contract for the servicing and MOT's for years 6 and 7 at a total cost of £691.70 paid over the two years. This does not include replacing the plugs as I expect the mileage by then to be only around 60,000, (Covid permitting!)
Gives an indication at least. I think its sensible to assume there will be deals to be had at Honda dealers, come the time. Moot point if you are trading in a 5 years, but if the cars still good to go, seems like it will be at least economic to keep longer