The plant in the photo is hibiscus syricus (Rose of Sharon). I have found that all hibiscus are finicky about environmental changes so much that even rotating a plant will cause leaf drop. Yellowing leaves equal stress of some nature. And If you do not see noticeable infestation, then this is a water issue. Most the time if lower leaves are yellow and dropping, it is a matter of over watering or soil compaction. I have great luck with a water retentive soil mixture with a healthy dose of added perlite and a dash of moss. I will top the recipe with mulch to protect the surface feeder roots. Hope this helps.
Ed

Thank you. Following your advice, I decided to add some water retaining content to the mixture and was surprised when I saw the mass of roots at the bottom. Do you think the pot is too small? I had read that hibiscus didn't like their pots too big so when I re-potted it, it was into a container not a lot bigger than the original. Thanks for your help.

I thought it might help to show you these leaves too.

Wow. The mass of roots is definitely an issue. I would move to a larger pot immediately. The 'pot size' advice only applies to tropical or hibiscus rosa-sinensis. This encourages blooming, and spring pruning of the roots is needed. As for your rose of sharon, they are varieties of shrubs and trees. I have seen 'diana' grow to heights exceeding 15 feet. I am not sure of your location and weather, but planting in the ground next year would be an idea. All of my altheas are in pots of 22" or larger.
Hope this helps.
Ed

Thank you very much for your help. Just one last thing - could I put it into the ground now? Here we're at the very end of summer, around 3 weeks from the first frost.

You may be pushing the envelope with the current state of the plant. It may take 2 weeks for it to recover from the move. Our winters can be similar and I over winter my perennial hibiscus the first year inside. If you decide the plant in the ground, make sure you use a lot of mulch and maybe even a bit of burlap to help with the cold.
Ed

OK, I'll put into a bigger pot and bring it inside for the winter and if it survives, put it in the ground next spring.
Thank you for your valuable advice.

You should be able to place it into the ground now. I have never had a problem with them wintering over no matter how cold our winters are or are not.

I wish you luck poitou. If you live in France, you had an exceptionally cold winter last year. If you need help this winter for your new friends, let me know. Remember 1-2 hours of sunshine and cut back the water significantly. Next spring they will be ready for pruning growth.