how did you find out about the bad reference? No offence but could there have been any truth in it? I am in a position where I both write and consider people's references and I have taken some training in it. We are told to stick to the facts, if anything undesireable has to be added, then ensure that it is objectively stated and absolutely proveable. If you wouldn't re-employ the person, then you say factual things like how long they have worked for you, what their responsibilities consisted of, anything that they have done well, and make no comment about anything else. We are asked to report on their sickness absence record and again we would just put the total number of days without comment unless the total is high but there were mitigating circumstances. Baseline is that while your manager may not have given you a good enough reference, she is unlikely (unless she is completely crazy) to have given you a bad one. If she did, unless it is objectively proveable, then your union should be able to help you