This paper describes the unit selection algorithm of a speech synthesis system, which selects the k-best paths over units from a relational unit database. The algorithm uses words and diphones as basic unit types. It is part of a customisable text-to-speech system designed for generating new prompts using a recorded speech corpus, with the option that the user can interactively optimise the results from the unit selection algorithm. This algorithm combines advantages of non-uniform unit selection algorithms and diphone inventory based speech synthesis.