Thanks a lot for all the suggestions - much appreciated. I'm definitely going to do this now - I'm booking my outgoing ferry today (that's the 'cast-in-stone' one to Rotterdam, from where I'll have the 2-day blast south via autoroute) and will then take my time plotting a nice route home, plus some rideouts from where I'll be based, in Montpellier. At this point I think I'm going to return via overnight ferry Caen-Portsmouth - the shortest run home in the UK and in effect combining the crossing with a night's accommodation. But we'll see...
The routes offered up so far have MOSTLY (not all by any means) assumed a return crossing via Calais.
France is a big country, twice the size of the UK with near enough the same population. Montpellier to Caen is roughly 550 to 600 miles, so there are several meandering routes you could take back to Caen, up to and including via Provence and Annecy if the mood and time allowed it.
Let’s though assume you want to go reasonably northwards. Putting Montpellier and Caen into something as simple as ViaMichelin’s website, will spit out three different ‘non-motorway’ routes. All three are pretty decent. The site has an added advantage that you can zoom in to see the detail, rather like having a paper map in front of you.
When zooming in, do take a note of when the motorway (which you are avoiding) does nothing more than track the course of the old D road, as in this example shown immediately above. The French obviously went to the huge expense of building the motorway. The reasons why vary but sometimes as it’s as simple as the old D road it replaced was garbage. It can sometimes make sense to hop on the motorway, at least for a stretch but that is up to you.
Likewise, bods on this forum swear blind that the only way to plot a route is to take the yellow D roads with a green border, as they are scenic. That is true, in as much as a fellow in the Michelin map maker’s office has decided that they are. This is fine but ‘scenic’ might not necessarily make them a “Great biking road, mate” as they’ll be just as ‘scenic’ for Fred and Dorris Bonkers, in their Honda Civic, maundering along. Likewise, look at a map, that ‘scenic’ road might take you through village, after village, after town, after village, after village, all jolly ‘scenic’ but possibly tedious if your real intent is to get to Caen to catch a specific ferry.
In other words, look at maps and decide for yourself if using roads A, B and C is definitely better than using roads, X, Y and Z.
One tip more. Michelin produce an excellent map, designed to answer questions like yours, ie. What is the ‘best’ way to go. It is map number 726. It strips out a lot of unnecessary detail, leaving just the main roads (including the very good BiS ‘tourist’ roads) which helps a lot. Highly recommended.