The law (I'm assuming you're in the UK) says basically that:
Service providers must make 'reasonable adjustments' to allow a disabled person to use their services. If they don't do this, they must be able to show that their failure to do so is reasonable.

To be regarded as disabled due to a sight problem, you need to meet one of the following criteria:
- certified as blind or partially-sighted by a consultant ophthalmologist; or
-registered as blind or partially-sighted by a local authority.

I'm guessing that you meet at least the first criterion if you've received treatment or the deterioration of your sight is being monitored.

My advice would be to write to the CEO of said bank and ask for his help. If you get nothing back from that approach, go to the local press, they love stories like that.
( Snappyuk gets BIGGER with free sample., Fri 24 Feb 2012, 18:39, 2 replies)

^This
with a big dollop of cream on top.
(Misery McUglywife an attention seeking sociopathic fuckstain., Fri 24 Feb 2012, 21:36, closed)

thanks
definitely going to try that, this needs sorting. the chip no pin thing seems like a good idea, too
( Smash Monkey lowering the tone of the whole internet, Sun 26 Feb 2012, 14:33, closed)