Preheat the slow cooker if necessary; see the manufacturer's instructions. Rinse the pheasant inside and out with plenty of cold running water, then pat fry with kitchen towel.
Heat the butter and oil in a frying pan, add the pheasant, breast side down, the shallots, bacon or pancetta and celery and fry until golden brown, turning the pheasant and stirring the other ingredients. Transfer the pheasant to the slow cooker pot, placing it breast side down.
Stir the flour into the onion mix. Gradually add the stock and sherry, then add the chestnuts, thyme and a little salt and pepper. Bring to the boil, stirring, then spoon over the pheasant. Cover with the lid and cook on high for 3–4 hours until tender. Test with a knife through the thickest part of the pheasant leg and breast to make sure that the juices run clear. Carve the pheasant breast into thick slices and cut the legs away from the body. Serve with potatoes dauphinois.