Theories of adult learning would be useful here.  Think of the adults currently UK universities, some at very advanced ages!  Personally, I like to avoid the word 'fossilization' with its negative connotations.  I remain endlessly optimistic that entrenched language errors can potentially be shifted, though it may not necessarily be simple or easy.  I clearly remember a piece of research that showed a piece of information needed to be revised at least 18 times before it enters the long-term memory (though I'm sorry, I can't recall the reference).  Perhaps some students cannot be helped very effectively: apparently atherosclerosis and cerebrovascular disease can affect memory and learning, plus some ESOL students are slow learners, and I often find that students have a high "affective filter".  That is, they have experienced so much trauma, mistreatment and sadness, that they are not, at least for the time-being, in a suitable state to learn, mentally and emotionally.  There are so many ways in which we can get students to 'notice' their own and their class-mates' errors.  A few can include finger correction; photocopy their unmarked written work and get them to check it; record their speaking, and get them to listen to it; grammar correction games; use the SMARTboard in various ways; make sure that students do not answer your questions instantly, but that they have a few seconds of thinking time before giving a spoken answer in class.  You haven't mentioned what errors you think are fossilised, but, for example, when it comes to writing, spelling and homophones, it is sometimes a good idea to teach spellings such as "there/their/they're" separately, not at the same time, as I think that is a sure-fire way to confuse some students.  I hope that none of this is patronising.

Best regards,

Philippa (Grimes)

(P.S. Ellie, thank you for sharing your really interesting paper!)