Need help! How can I fix my "anti pollution fault" on Peugeot 307?

I know this question has been answered a number of times on this site quite extensively however i couldn't find the answer to my problem and i'm hoping somebody can help me out.

I have a Peugeot 307 2.0 HDI (2001) and recently the "anti pollution fault" light came on my dash. I took it to my local peugeot dealer who charged me £30 to "link up computer" to the car to investigate what the problem is and they have told me I need a new "throttle potentiometer" fitted to the tune of £150. Now i'm no expert in cars but i've read up on a throttle potentiometer and it seems to be directly related to the cars power output which I am experiencing no affect on whatsoever so i'm thinking maybe peugeot of diagnosed the problem incorrectly.

The car's performance is not affected whatsoever but the light comes on constantly now from the moment I turn it on.

I haven't left the car back in yet but hope to do so soon and was wondering if there was any advice anyone can give please...anything at all would help?

Many thanks in advance!