A Senior Lecturer in a post-1992 or "New "University (former Polytechnic, mainly teaching) is equivalent to Lecturer B in an "Old" (mainly research) University.
An Senior Lecturer in an Old University is roughly equivalent to Principal Lecturer in a New....
Assuming you are interested in old, an eg of critieria and processes, from University of Manchester, is here:
http://www.campus.manchester.ac.uk/medialibrary/policies/promotion-senior-lecturer.pdf[%sig%]