Misc: One light year is equivalent to how many million kilometres?

Questions

Who established horse power as a unit of measurement?

What was the eventual title of a book which was originally called Incident at West Egg?

Robins Nest and George and Mildred were spin offs from which sit com?

Which London theatre house is built on the site of a well once famous for its medicinal properties?

By what more familiar title is the First Lord of the Treasury better known?