when asking for a reference they usually ask for a referee, if there is anyone you feel will give you a positve reference it may be good to ask them.
It would be worth your while to ask the hr department of this firm and ask what would be their poicy in this matter. Some firms will only give a basic reference saying you worked there, what position and the start and end dates.
A few years ago my daughter got a job and started work. Three weeks later they asked her to leave because her previous employer had given her a bad reference (what they said wasn't true). She went to see a solicitor and he said there was nothing she could do about it!
Ethel, it was just before that case and didn't involve disability. They told a complete pack of lies about her timekeeping and being off ill due to vomiting and diahorrea (she worked handling food!).