The amount of discount would depend on the type of work discount you are entitled to. You do not say what work discount you are opting for. This information would assist us in advising you further.
Code Check and confirmation process: You can text OPEN followed by the company name to 61202 and it will confirm your eligibility and the O2 Open code to use. Make sure you try different variations of your company name if unsuccessful with one version. ie, NHS, health service, health trust, etc.