So if i said that i believed it to be true that a generic united player was a rapist and murderer i am ok
You would be ask to explain why you believed it to be true. If you could justify that belief, then yes. You would have a hard time justifying that without evidence that supported it.
If you said a United player was a 'bit of a wanker' - you'd have more justification and you'd be expressing an honest opinion.
If you try to assert something as fact, and you can't back up why you believed it to be fact, then the courts would probably assume you knowingly lied (thus didn't didn't reasonably hold the belief in the first place).
If you assert an opinion, it's a different matter, although you should still provide decent reasons why it's your opinion.
You can't say 'in my opinion - so and so is a murderer and rapist' and expect to get away with 'in my opinion' though - not unless you have some decent evidence to support that opinion. An example might be if a case rape / murder trial went to court, and the defendant was found not guilty. You MIGHT be able to justify a belief that the courts got it wrong, and that the defendant was indeed guilty, but even then, you'd be treading on very thin ice.
However, if a bloke in the pub tells another bloke that you've been having affair with his wife, and he reasonably believes that to be the case, you'll have a hard time getting any damages.