Help with A3

A mate of mine has a Graham Farish A3 Prince Palatine. We're trying to get it apart for cleaning but can't find a way in. We've taken all the screws out from underneath, that we can find, and had the keeper plate out, but still can't get the body off. We can see the end of a screw thread which is obviously been put in from above, but there's no hole down the chimney to access it. We'd appreciate a bit of help before we go nuts.