Heat the oven to 180C/fan 160C/gas 4. Bring the guinea fowl up to room temperature while you prep the veg. Heat the butter and oil in a pan and brown the guinea fowl all over, then lift it into a casserole in which it will fit fairly snugly (leave the legs tied). Add the pancetta to the frying pan and brown it, then tip into the casserole.
Do the same with the shallots then the carrots and celery. Tuck the bay leaves and sage around the bird. Heat the vermouth in the frying pan, scraping up all the flavour that is stuck to the pan. Pour over the fowl (this should help wash the veg down around the bird). Put a lid on the casserole and cook for 40 minutes. Untie the legs and put the casserole back in the oven without the lid for 10 minutes.