Thanks for the suggestions, we're both currently looking into panniers for the France trip.
Most of the ones we've seen are huge and look a bit daft on our bikes.
In the photo, there is a lot of unused space, but this was probably taken on a day when the tents were already off the bikes. The yellow bag you can see is actually a 50l waterproof pack liner, so there's plenty of room in there to allow for extra kit. So long as it fits under the cargo net.
One issue we still have, is being able to get off the bikes and wander around some of the French cities. It's not a problem in the more rural areas, but when we're away from the bikes for a long time in cities, we'll need an easy way to take the gear with us for security.
For this, I like the idea Mortis suggests of a rucksack strapped to the pillion. Again, so long as it can be made secure. I imagine it will be heavy and will stick out a fair way each side. I guess it's another thing to add to the list of tests to carry out on our test ride in a couple of weekend's time.