During the early stages of hot commissioning of the Grimethorpe Experimental Facility it was found that the Distributor Plate Seal did not operate correctly. It allowed the upward leakage of fluidizing air and the downward leakage of bed material out of the combustor. The seal was modified and has now performed satisfactorily during a number of hot commissioning trials. Additionally, a further modification involving the elimination of the seal from the base of the combustor is being developed and will be available should the first modification prove to be unacceptable as a long term solution.