With thanks to Rovastar, I can't believe I missed off some service roads from my recent mappings; I'd mapped some parking areas, but not connected them to any highways, leaving them in a kind-of "limbo".
For some reason, I'd got it into my newbie head that I didn't need to map "across pavements", even though I could see the new ways I'd created didn't link to a "main" way...
Must be the medication... ;-)
Anyway, Rovastar very kindly added them in for me on one example (as below), and I've now gone back and added them in on some other mappings I've done and missed them from.
Great advice from them too; go and look at another example of something, before mapping a new instance of it yourself, as you can pick up lots of tips from other people's work.