Robert Simpson's Third Symphony (1961-2) marks a transition between the two stages into which the composer's oeuvre is generally divided (as one of his last works based upon major/minor keys, before his conscious composition with intervals). It is unusual within Simpson's output in three respects: it incorporates models, from Beethoven; its genesis can be traced in two manuscript autograph scores and two published versions; and the autograph scores preserve unique revisions by the composer. The issues that arise in consequence are considered in the context of general influences upon the symphony, a description of the manuscripts and published scores, and remarks about manuscript interpretation. From an analysis of the modelling of the first movement upon the first movement of Beethoven's Ninth Symphony, a reconstruction is offered of music lost from the earliest stage of composition. This reconstruction permits the elaboration of a hypothesis with respect to an atypical feature of Simpson's style: Simpson's revisions were most far-reaching where Beethoven's influence had initially been strongest. Further Beethovenian influences are identified, with their bearing on the manner in which Simpson revised and utilized the typical features of his style which are fundamental to the Third Symphony's symphonic structure: rhythm and pace, tonality and harmony. The influence of these revisions upon Simpson's Fourth and Fifth Symphonies is also investigated. With reference to the Beethoven influences identified earlier, parallels are drawn between developments in Simpson's style, and Beethoven's between his 'second' and 'third' periods. 'Suggestion' (a process used by Simpson, of statement 'by implication', related to compositional compression in the revisions) is discussed and considered as a metaphor for the development of Simpson's style as Beethoven's influence upon him weakened. A brief comparison of Simpson's Third Symphony with his Fourth and Fifth Symphonies is also included.