Took about an hour to drive (including the new bit of R747) and nearly two hours to trace (I'd swear there's not a single straight bit longer than about 300 metres in the whole 27-ish km each way), but it's pretty much done. A few missing details such as whether some of the back roads go over or under the motorway. I'm missing traces for southbound through J3 (I came off at that point) and northbound through J2. Also need through-traces for J4 in both directions, but it's easy enough to guesstimate close enough.
Will upload my GPX traces tomorrow when I'm less tired.