cannot select python mode manually

[Ray]

A bit of an EMACS newbie here, despite having used it on and off for years. 
So forgive me if this is really simple!

If I edit a python file the python mode comes in automatically and works
fine.  However,if I have another file and want to engage python mode
manually (M-x python-mode) Emacs replies [no match]

python-mode.el and .elc are in /usr/share/emacs/site-lisp along with other
modes that work fine when invoked with M-x !

Can any one give me some pointers as to what to check?

Cheers
-- 
Ray