That's exactly the kind of thing above happening with me Nigel, glad you posted that. Strange thing is, get similar behaviour from Garmin Connects own mapping program.
Its just a matter of checking your route before saving it. I know most of the roads I ride around here now, so I just use it for a guide and for distance purposes.
I never use the Garmin plotting, you just get used to what works for you I guess.
I dont really need full mapping, I do have a tongue in my head if needed. I can navigate by the sun (what sun you might say) and stars if need be, I know roughly how the road number system works. I pretty much know the layout of the country ( I used to drive a truck, and have lived from Kent to Skye, though usually Manchester Leeds area of the country).
I would like an 800, but I dont think I would really get much use out of the mapping, an option is the Bryton 50, I know the 35 was more accurate than the 500, but it's breadcrumb trial left a lot to be desired.