Can i renew my schengen visa in Italy or do i go back to the consulate in manchester to do this?

A Schengen visa cannot be "renewed" in a Schengen country. You are supposed to leave the Schengen territory by midnight on the last day your visa is valid or when your number of authorized days of stay are used up. A visa extension in a Schengen country is only possible for humanitarian grounds like severe illness with a doctor's statement that repatriation is not possible for medical reasons or a sudden death of a close relative you have visited with the funeral to be just some days after the expiration date of the visa..

Renew Schengen Visa

You can do it in the consulate in Italy. Yahoo search where it is.