According to that and another paper I found, the timing of migration is temperature dependant. In colder years they move away earlier [and my experience concurs with that over the past couple of cold ones]....so, with the mild weather this winter, I hope they stay a bit later [i have had good catches in early Feb., but never as late as the school half term hol. in mid Feb]. Hope to give it a couple more goes [with a bit more confidence].