A few days in the Dordogne would be in order. Check out the Michelin Green Guide for that area, or Fodor's for that matter

Spend a night in Carcassonne. I really enjoyed walking around in the evening after the tourist buses had left. I stayed in the Best Western Hotel le Donjon and it was pretty good.

Thanks Michael
After some very preliminary investigations I've come up with the following "draft" itinerary. Any comments would be much appreciated:
1 night in Amiens region
3 nights in Paris
2 nights in Loire Valley region
head toward coast maybe 1 night in La Rochelle
3 nights in Dordogne based at Salat
2 nights through Pyrenees/South
Drop car in Perpignan and train into Barcelona
Am I mad or does this look "doable"
Many thanks

Any reason why you are using Amiens as your starting point? Starting the French part of the trip in Paris might be better.

We were coming from London and thought (not based on any research!!) we could cross at Dover, see the areas (Amiens/Fromelle) etc relating to WW1 Australian diggers and then head down to Paris as opposed to heading back North then South again.
Appreciate your thoughts.

While September is generally recommended as good month for the Pyrenees, you still need to be flexible about your driving plans, and watch local weather forecasts for thunderstorms. Even without storms, you need to be watchful of clouds, because what merely looks like clouds from below turns into blinding whiteout fog as you ascend into them. Some of the higher elevation drives are already steeply gradded switchbacks without guardrails in clear weather. Not where you want to be with zero visibility!

If you need to abruptly change your routing, there is plenty to enjoy at lower elevations in that region and no real need to book anything in advance. There is a huge amount of accommodation in the Pyrenees sitting empty in September, and likewise at lower elevations. So don't lock yourselves into anything and be prepared to turn back if need be.

You really won't the car in Paris ...traffic, parking etc.

I meant to say "won't WANT the car in Paris"