Hi sherrard -my mum is 86 and this is her recipe for the filling
Peel and dice about 1/2 inch cubes 3 medium potatoes and steam for roughly 10 mins - chop/mash up 1 can corned beef place in large bowl -finely chop 1/2 sweet onion -raw -and add to corned beef -add hot steamed pots to corned beef and stir well but do not fully mash. Add seasoning to taste leave to cool and then fill pie. Cook in hot oven until pastry is cooked -about 30 mins. Serve with gravy and peas.