Some colleges allow you to extend your contract, especially since they know a lot of peoples private housing contracts usually start in July when move out days happen in June. There are a lot of tenancy takeovers for the summer being advertised on the 'Overhead at Durham university' or look for a housemate group on FB, otherwise - some in student halls with some great discounts.
Student halls in Durham don't usually do summer contracts as most of them don't offer contracts less than 49-52 weeks, but people moving out for the summer will do tenancy takeovers instead.