Dumpertruck - print off a copy of the eyesight requirements from the CAA website, take it to an optician and get them to do a full opthalmologists report (any optician will do this for about £15-20) which you then send to the CAA Medical Division who will have a look at it and tell you whether it's worth you turning up for a medical or not. Surprisingly, the CAA will actually do this for free!! It's probably the only thing the CAA will ever do for you for nothing, so take advantage.