Meanwhile, heat the oil in a non-stick frying-pan over a medium-high heat and add the ling fillets, skin-side down. Cook for three to four minutes until nicely coloured, then turn the fillets and add the rest of the butter to the pan. Fry for a couple of minutes until the fish is just cooked through. To serve, spoon the creamed peas, leeks and bacon on to warmed plates and place the fish on top.