Thanks to all. Ten months after writing that post, there have been some developments. In July 2014 I began attending the Melbourne Headache Centre (http://www.melbourneheadachecentre.com.au/) which treats patients on the theory that migraines in many people are caused by an overactive brainstem. This involved twice-weekly half-hour consultations which involved manipulating the C2 cervical vertebrae in my neck as well as specific neck stretching exercises several times a day. At that stage my migraines were recurring at intervals between 3 and 6 days.
In September I ceased using the Endep (Elavil) in order to assess whether that Headache Centre treatment was having any impact. After a month the headaches were retreating to about days apart. In late October I opted also to abstain completely from wine (I still drink beer)> Since then I have had only three migraines, with the last one now almost seven weeks ago. The last few have been quite mild, rarely developing into anything at all once I take Imigran at the onset of the aura.
I think the neck treatment (and work on my posture and neck pressure) contributed, and probably the absence of wine has also had a major impact. Then again, who knows? But I am convinced that neither the Endep nor the beta blocker had any preventive effect at all.