Decide who is going to write press releases, or give your local newspaper a call and ask to speak to someone on the newsdesk and let them know what you are doing and when.
Try and go for an unusual angle when dealing with the press. Not "X received a cheque for X pounds from X and here's a photo", but "X abseiled down the council building and presented a cheque for money the staff had raised in a marathon tug of war – photo of tug of war".
Use any contacts you know to spread the word of your event: family/ friends may be willing to put a poster up at work or perhaps sell a few tickets for you, for example.
Publicity prior to an event can help sell tickets. Publicity after the event raises the profile of the charity.
Information
How will the money be spent? What does Diabetes UK do? What is diabetes?
Of course you know, but does everyone taking part in the event know? You need to be prepared to answer questions from friends, relations, the man or woman in the street. Include this information in your publicity when running anything from a coffee morning to a fancy dress hockey match to a barn dance.