The magnitude and seriousness of the challenge posed by heterogeneous multimorbidity in most health services is now unquestioned. Equally well-acknowledged is the fact that existing guidelines essentially set out the principles of best practice in an idealised setting, concentrating on information gathering and not providing any personalisable decision support for the general practitioner aiming to share decision making with a person with multiple morbidities in the reality of routine practice. Existing decision aids have been developed largely within the single condition context and can draw on a body of robust research largely absent in the multimorbidity context. The need for a more flexible and generic approach which draws more on the clinician's expertise-based judgments has been called for, and we introduce a Decision Support Tool, GREST CLIN, in response to this call. Based on Multi-Criteria Decision Analysis it reflects the quantitative calculation approach to decision support rather than the deliberative reasoning of mainstream decision aids. It is emphasised that any evaluation should use the current decision making process as empirical comparator, rather than idealised normative standards, and that the primary outcome measure should be decision quality at the point of care, not downstream outcomes. A demonstration version of the tool is available online as proof of method.