What you have is a penalty notice for disorder. The fine would have originally been for £80 but since you have not paid it within 21 days, the fine goes up by 50%.
If you pay the fine then that's the end of it. However if you don't then a warrant may be issued for your arrest. If you do not want to pay the fine because you have not committed this crime then you can request a court hearing, but I think this time may have expired since you have not done anything within the 21 day limit. I cannot remember the specifics.
Moveing onto CRB checks, if you pay the fine, the only CRB check that MAY show the fine is an enhanced CRB check. If you pay the therefore you have no need to worry about anything. The fine will not show on a basic or standard disclosure.
Paying the fine is not an admission of guilt and does not constitute a record of criminal conviction.
You have left this rather late and for this reason I would really urge to speak to a lawyer before this turns bad and you find youself in court. If you are convicted of this offence then you will have problems.
Pay the fine and that's that.