Cut a cross on the bottom of the tomato. Bring a pan of water to the boil and blanch the tomatoes for 20 seconds until the skin starts to peel off. Refresh in iced water and reserve the peeled-away skins.
Cut each tomato in half , scoop out the seeds and set aside then cut each half into three pieces. Place onto a tray and season with salt and freshly ground black pepper. Sprinkle over the thyme and drizzle with two tablespoons of the olive oil.
Place the tomato segments into the oven and cook for 1½ hours. Turn the oven off and leave the tomatoes for at least four hours, preferably overnight to dry out.
Shell the prawns, reserving the shell and head. Heat a frying pan until hot, add the remaining olive oil and half the butter. Add the prawns and fry for 2-3 minutes, or until pink and just cooked through. Remove from the pan and set aside.
Add the shallots, garlic and prawn shells and fry for 2-3 minutes.
Carefully pour in the vodka and flambé the liquid by slowly tilting the pan towards the gas flame, or light it with a match. Let the flames flare up then die down. (Caution: keep your face and hair away from the flames. Do not leave unattended.) Add the reserved tomato skins and seeds and fry for another two minutes.
Pour in the fish stock and cook until reduced by half. Pass the sauce through a sieve into a clean saucepan, pressing down on the prawn shells.
Add the cream and cook until the sauce has thickened slightly, then add the prawns and warm through. Season with salt, freshly ground black pepper and lemon juice. Stir in the dill and chervil.
Heat a frying pan until hot, add the remaining butter, samphire and 75ml/3fl oz water and cook for 2-3 minutes, or until tender. Season to taste with salt and freshly ground black pepper.
Spoon the prawns and sauce onto a plate and serve the roasted tomatoes and samphire alongside.