Ignition switch

["g.cook" <litfuse@triton.net>]

My 1980 MGB has a problem with the ignition switch: the key(s,
including  original and copies) refused to turn.  No warning.  I
eventually got it to work with WD40 and heating the key over a flame,
but it seems inevitable to stick again.  Moss would have me replace the
whole assembly, which strikes me as Miatish.  (I just made that up.) 
Anybody else had this problem?  Would a locksmith be able to repair
crucial parts?