Peel the prawns, keeping the heads and shells, and devein.
Add some oive oil to a deep, heavy based saucepan and add the prawn heads and shells. Fry on a medium heat.
Roughly chop a garlic clove, half the fennel, the celery stick and half the onion. Add to the pan with the prawn shells and cook until the veg starts to soften. Add the white wine and let it bubble away. Then add the tinned plum tomatoes and a tin of water. Bring to the boil and then gently simmer for 15-20 minutes.
In batches add the prawn shells and sauce to a blender and blitz until qute smooth. Pass though a fine seive. Discard the solids and keep the sauce to one side.
Wipe clean the saucepan and put back on the heat with some more olive oil. Finely slice the rest of the onion and fennel and the remaining garlic bulb. Add to the pan and sweat gently for about 10 minutes.
Meanwhile par boil you new potatoes in salted boiling water for about 10 minutes. Then drain the potatoes and add them to the vegetables. Pour the sauce into the pan and let it bubble until the potatoes are almost cooked.
Cut each gurnard fillet into large chunks. Add to the pan with the prawns. Cook for 4-5 minutes until the prawns are pink and the fish is opaque and flaky.
Cook the samphire in salted boiling water for 2 minutes. Dish up the stew and top with samphire, a squeeze of lemon juice and saffron aioli. Serve with crusty bread.