It is a good idea to speak to a trade union rep if you belong to a trade union, or take advice from a specialist charity.
The disability discrimination provisions of the Equality Act 2010 state that you have to show that, on or before the date of the negative treatment, you had "a physical or mental impairment" which had "a substantial and long-term adverse effect on your ability to carry out normal day-to-day activities".
The Equality Act does not include a list of impairments, but the statutory Code of Practice for Employment (PDF, 1.1MB) produced by the Equality and Human Rights Commission, includes more information. Court cases have applied disability discrimination protection to a wide range of conditions including back disorders, depression, asthma and ME (chronic fatigue syndrome). 'Mental impairment' also includes learning difficulties.
Long-term means that the disability must have lasted, or can be expected to last, at least 12 months. People with a fluctuating condition that is likely to recur within 12 months are covered. People with progressive conditions, such as cancer, multiple sclerosis or HIV, are included from the date of their diagnosis. Any kind of cancer is protected, even if it is non-invasive and unlikely to get worse, for example some types of skin cancer.
There used to be a statutory list of normal day-to-day activities, but this has been abolished. Instead, in any tribunal claim to enforce your rights you would need to produce evidence that your own normal day-to-day activities are adversely affected.
People who have had a disability within the definition continue to be protected from negative treatment linked to their past disability, even if they have since recovered.