I Am Trying To Find The Title Of A Group Of Books I Read In Junior School (8-11).?

the books main theme was witches,a magic drink(which i think made the boy invisible)trees that came alive etc.the only name i remember was for a cat called Tabias who helped the boy(main character)on his adventures.each book continued from the next,and i read these at junior school so maybe they were only in schools as part of the curriculum.i was at junior school in 1978-1982 in the loughborough area of leicestershire.thanks for any help.