Legal question

If I map an area as best I can with a satnav and then use to overlay OSM on top of G aerial view and then tweak my placement of ways etc to match, does that count as a derivative work?

I think legally, the answer is “maybe”. Of course its depending on where you are in the world, and just how much you copy from the aerial images. Also Google’s terms and conditions, which may forbid this.
Richard Fairhurst has an interesting blog post about this here (and worth reading some of the comments):

But in OSM terms, the simple answer is its not allowed. The point of OSM is to make a a map that is definitely free of copyright restrictions, so avoid anything which might be of questionable legality. See the Legal FAQ:

Anyway, how do you know that the Google images are more accurate than your GPS traces?

The creativity/sweat of the brow/uncharted legal territory is in the distortion of the photos by Google or their provider. Unless the land is perfectly flat, or the photo is taken from directly above the location, some transformation is needed to take terrain into account. Sometimes when recent highways have created cuts and fills that are not in the elevation dataset they used to do this, the roads will look wavy because they do not follow the old terrain. Here’s a relatively mild example; the street view shows that it’s actually a continuous curve through that cut.