1. Speech synthesis apparatus comprising: a plurality of synthesis engines having different characteristics and each comprising a text-to-speech converter for converting text-form utterances into speech form; a synthesis-engine selector for selecting one of the synthesis engines as the current operative engine for producing speech-form utterances for a speech application; and an assessment arrangement for assessing the overall quality of the speech-form utterances produced by the current operative synthesis engine whereby to selectively produce an action indicator when it determines that the current speech form is inadequate; the synthesis-engine selector being responsive to the production of an action indicator to select a different synthesis engine from said plurality to serve as the current operative engine.