All reserves must be reviewed as part of the annual budget-setting process. Perhaps you could propose that the council publishes a policy statement on this reserve, explaining exactly what it would and wouldn't be used for. For example, would you challenge 10 dwellings, or 30, or 50? Is it for specific locations or types of land? Is it exclusively for development within the parish boundaries, or would the council join with neighbouring councils to contest development elsewhere? How would you define the detriment that is required to trigger using this reserve? Would you challenge projects of national significance (HS2, airport expansion etc)? What services would you expect the consultant to provide in each scenario and at each stage of the process?
By bringing all of this out into the open, you might split opinion sufficiently for the council to decide that the reserve has no meaningful purpose. Conversely, of course, your colleagues might decide to double the amount held!