Bring a large pot of salted water to the boil. Peel your potatoes, chop them into even sized chunks, boil, drain and mash.Into a bowl put crab meat, prawns, spring onions, mushrooms and diced panchetta. Add the egg and chopped coriander with a really good pinch of salt and pepper. Finely grate over the lemon zest, then mash and mix it up really well. Dust a plate with a little of the extra flour. Divide your cakes into 4, lightly shape and pat into circles about 2cm thick, dusting them with flour as you go. Put them onto a clean plate also dusted with a little flour. Put a large fry pan on a medium heat and add a couple of lugs of olive oil. When the oil is nice and hot, add your cakes and cook for about 3 – 4 minutes on each side or until crisp and golden.