Lay the slices of pork fillet between two sheets of cling film and flatten using a rolling pin to a thickness of about 1cm/½in. Season the flattened slices of pork with salt and freshly ground black pepper.
Heat a knob of the butter in a frying pan over a medium-high heat. Add the pork slices (in batches if necessary) and fry for 1-2 minutes on each side, or until lightly browned and just cooked through. Remove the pork from the pan and set aside on a plate. Keep warm.
Add another knob of butter to the pan and fry the shallot for 1-2 minutes, or until just softened. Increase the heat to high, pour in the Armagnac, then set the alcohol alight using a match. Allow the flames to flare up and then die down. (Caution: Do not allow your face or hair to get too close to the flames. Do not leave unattended.)
Add the stock and 150ml/5fl oz of the cream to the pan and bring the mixture to the boil. Reduce the heat until the mixture is simmering, then add the prunes and return the cooked pork and any pork juices to the pan. Continue to cook for a further 2-3 minutes, or until the pork is completely cooked through.
In a separate pan, heat the mashed potato with the remaining butter and cream until warmed through and well combined.
To serve, divide the mashed potato equally among four serving plates. Place a few slices of the pork onto each serving plate and spoon over the sauce.