Tea, thanks for your reply. As a rule I avoid googling any medical condition as it's liable to send one into a panic, convinced that you have only minutes left to live! I trust that your referral went OK?
Loose Rivets, thanks also. I did go to see my GP shortly after posting, and she didn't seem too bothered and said it wasn't worth referring me to a specialist (she was also pretty nonplussed with my impersonation of an altimeter). I now wish I'd been more insistent on asking for a referral.
Your 'alternative' explanation of the altimeter needle thing is very interesting. Next time I'm in the cockpit I'll bear it in mind and will see if the picture fits, so to speak.
Either way I think I'll have this looked into again. I can certainly live with it as it has no practical impact whatsoever on my vision, but better to have this confirmed, I think.