The last two posts are correct - people are entitled to give a bad reference if it is totally factual and fair - in fact if they lie about an employee who underperformed, and give positive statements which they know to be untrue, they can be sued by any employer who relies in such a statement. This is why so many companies just provide the very barest details and do not give any opinions at all on a reference.
In some jobs employers though have to giv a full reference. Childcare is one of them. That is because the reference is one of the key tools and without it, a person seeking a job is unfairly disadvantaged against other candidates.
There is a duty of care owed also to you that the reference is not defamatory or untrue. In practice this means that if they are saying bad things about you then these things should always be things about which they have already discussed their concerns with you during the course of employment. Therefore if you always turned up late and so did lots of other people and you thought no-one minded then they may be in hot water if they mentioned it. But if they had already pulled you up about it and told you not to be late then you would have no complaint if they mentioned it in a reference.
So, if you have ever been disciplined about anything or had poor appraisals, you can assume that these may well be in the reference and you won't be able to do much about it
You have a right to see a reference under the Data Protection Act (you need to make a request stating that you understadn that you have a right to see this reference) but that doesn't mean that you will see it - if it is a defamatory statement then chances are they will refuse to give you a copy and you will have to get lawyers involved and may end up in court.
If you feel that something is really amiss I would try to secure a role 'subject to references' and then see if the job offer was removed due to the reference and if so ask the employing company if they can explain what was wrong on the reference to get an idea - they may not tell you but you will probably get a much better feeling as to whether your instincts were right. If they were then you need to get a lawyer to right to demand to see the ref under a Data Protections Access Order (I think that's the term)