Mik
When I had my test the GP did my eyetest which included a field of vision test. He basically 'wiggled' a pen while moving it into my view and I told
him when I could see it.
The eyetest they were looking for then was simply to ensure I wasn't visually impared at all - they are not supposed to be offering you a prescription.
It's just like the one in your driving test - unless they've changed that too
You need to speak to Sheila Barter at the MSA, she is the Medical Co-ordinator and her number is 01753 765000 (switchboard). She isn't in today

but should be in on Monday.
NiG
You could also try this link...
http://www.msauk.org/Go_motorsport/doctors.htm