Thank you all for taking the time to reply to me.
Dexter is not under a behaviourist, I haven't gone down that road yet. He doesn't have all the symptons of SA, i.e following me everywhere (I have gradually built up his 'time on his own' when I'm at home, so much so that he is happy to be apart from me for an hour or so). He also doesn't get stressed and anxious as I'm about to leave, in fact he quite happily gets into his pen. He is always walked (at least half an hour, twice a day) I do some training exercises most days, he has toys, chews and stuffed kong, dap diffuser, radio on, I don't fuss him before I go or when I come back in. The time he is left varies from an hour to a max of three hours. I just think that he doesn't like being an only dog, and I am in the very lucky position of being able to get another dog (we're moving to a bigger place with a large garden soon).