Does anyone know how old my wiring is likely to be? What is the
legal position on earth wires in lighting circuits?

The lack of earth isn't much of an issue, especially if you avoid metal
fixtures. The rubber issue is much more serious. It has an expected 25 year
life and hasn't been installed for 40 years. Obviously, it lasts better in
some cases and worse in others, but it should need replacing by now. How
urgent it is might be determined by an insulation resistance test, although
even if it passes, it should really be replaced. You could always install
wall lighting in every room. ;-)

Christian.