I don't think an employer isn't allowed to give a 'bad' reference, if they don't want to write anything nice about you then they can merely write things like '**** was employed within the company from ????-????'
Don't quote me on that but its what my manager said his legal advisor told him when he wanted to write that an ex collegue was always late, off ill and had no respect for authority (apparently classed as slander) :s
oh dear...i'm never late, was off ill 3 times in 5 years..2 times being one day each
then last year with a bad chest infection, took 2 days then strugled back in.