Thanks everyone. Well, every website has its own favourite cause/solution from lurgy to age to diet. The most popular first steps that I have found are to keep the nail moisturised and protected, especially at night to stop it catching when asleep, so they will be done.
This morning I have rubbed some cream into the nail and noticed a few things. There is a dip by the root/half moon, it is a valley rather than a ridge. Also, if I squeeze the sides of the nail it does not bend in a nice arc, as the others do, there is a definite kink where split happens. To me they indicate nail bed damage.
I have to go to the doctor's in the next two weeks for my repeat prescription so if it is not improving I will make an appointment.
ETA Aggers, thanks for the link...... though I reckon that I can cross 'menopause' off the list of probables.