UPDATE: Spoke to the vet again today as the diazepam is making no difference and Gus' head shakes seems to be getting worse. My vet feels we should try phenobarbital (which I have at home because Pippin is on it for epilepsy) for 4 or 5 days as he feels Gus may be having focal seizures and I've to talk to him again on Friday.(or sooner if I need to)

I have checked out focal seizures online and some of them do look like what Gus is doing...would this be caused by SM?