Thanks everyone, really appreciate your interest and comments. Excuse the long reply coming, but I wanted to respond to some of you.
Pansyface - no, we don't, and we are on a hill outside our nearest village, so have no 'visitors'.
Fidgetbones & Lily Pilly - I really don't think it is blight, I have scoured the internet looking at pictures of blight and this just doesn't resemble it. There's plenty of leaf on the plants, they're just this peculiar colour. Also, blight is so infectious whole swathes would be affected, which is not the case.
Judith5, it gets a yearly dose of a local product, Bleu Bordelaise, from our gardener, but even our olive trees get the same dose, so I think perhaps I need something stronger/more specific.
Wintersong - I too am leaning towards a nutrition problem. Could you please let me know the names behind the initials in your response, so I can try to find something similar locally or online? This part of France, because of the extremes of the climate (minus 16 last winter and approaching 40 in the summer), is a bit behind the times in gardening as an art, although things are improving as irrigation systems become more the norm.
Nutcutlet - you're absolutely right, the worst ones are due on a bonfire this week, and I will replant.
Wintersong means Bonemeal or Bone and Fish meal. You can buy tubs of this and spread it around the Box plants. It is a general feed for this time of year and will help.
I would try that and some grass feed (high Nitrogen) before removing any box. I also agree that it is not blight!.
Excellent, thanks Blairs. I will start a trawl of my local nurseries. I do love living here, and I love our garden, but it is a total pain sometimes trying to find equivalent products!
Bonemeal is Poudre d'Os and is good for promoting strong roots. I have no more Blood, Fish and Bone so can't give you the exact French translation. However, if you can get it, pelleted fumier de vache, poule et cheval will do the job too and is clean and easy to handle.
Bleu bordelaise treats algae and fungal infections and is not a fertliser. I would give your box plants a generous helping of the slow release fertilsiers mentioned plus an instantly accessible liquid tonic of engrais liquide pour tomates.
You can also make your own liquid feed for greenery by soaking nettles in a bucket of water (with a lid on for the smell) for a couple of weeks and then diluting the resulting nitrogen rich stew 10 to 1 with water and using as a foliar feed by pouring it over the plants. You can do the same with comfrey and the resulting stew is good for flowering and fruiting plants because it has a wider range of nutrients.
sulphate of ammonia is a good high nitrogen fertiliser. I've used it with box and bay for years.
I don't think lack of water can be the problem, as we have an automatic watering system in place. Box is used a lot here in Provence as it doesn't require too much of it. I have orange box plants cheek by jowl with lovely dark green specimens in a mixed border.