...reading all the above myself, it's looking like while Ridewithgps recognises any type of track/road/path/whatever that can be part of a route, MyDrive will happily recreate the route from the gpx route to view, but when it syncs with the TomTom, if the map doesn't recognise the 'off the main road' element, it ignores it and 'smoothes' it out...
Yes, that is how it appears, but it should not do that. If there are track points set in 'legitimate geometry', the 550 should route through that bit of map geometry and not around it. That they are legitimate, at least as far as MyDrive Web goes, is what we're about to demonstrate for this ticket in this post.
Understand, however, that what MyDrive web shows are the results of the raw trackpoints that were uploaded in the *.gpx, which it, and the device, may or may not consider a viable routing for one reason or another. However, as the MyDrive screen shot below demonstrates, that's not the case for MyDrive web. It allows a waypoint to be added in the lay-by on this short little eastbound route. That's what makes this case an easy one to use to demonstrate the issue, so I'll just work with this one.
Next, we'll want to know what happens if, on your 550, you plan a short route from 52.59422, -0.67579 to 52.59370, -0.65220 and then add a stop at 52.59444, -0.66691 at the lay-by. Doing it that way, what does the 550 show? If you're not sure how to enter locations on your 550 with coordinates, it's done by clicking the "Whole map" button in the upper right of "Search" and scrolling down to the bottom where it allows for entry of lat/long data.
Send along a pic of what the 550 does with that. I want to exclude road type issues in the 'Classic' style map on your 550 from the discussion that is bound to follow with TomTom. I'm trying to avoid a lot of unnecessary questions back and forth so that we can get to the actual issue more quickly.