Preheat the oven to 200C/400F/Gas 6.
Sift the flour and salt into a bowl. Divide the butter into four equal pieces and put three back in the fridge. Rub the remaining quarter of the butter into the flour until the mixture resembles fine bread crumbs.
Add 125ml/4fl oz of water and bring it together to form a dough. Do not knead or over work the dough.
Wrap the dough in cling film and place in the fridge to rest for 15 minutes.
For the filling, bring a pan of water to the boil. Lower the eggs into the boiling water and immediately set a timer for two minutes and 45 seconds.
Remove the eggs with a slotted spoon and place into a sieve. Put the eggs under running cold water for a minute and then place in a bowl of cold water for a few minutes.
Peel the eggs and return them to the cold water and set aside.
In a medium sized frying pan, fry the mushrooms in a knob of butter until golden-brown and softened.
In a food processor, blend the sun-blushed tomatoes to a purée. Stir into the cooked mushrooms and put the mixture in the fridge to cool.
Remove the pastry from the fridge along with one of the remaining three pieces of butter. On a lightly floured surface roll out the pastry to form a long rectangle.
Dot the butter over the top two-thirds of the dough. Fold the bottom third over the butter and the top third over the top (like a letter ready to go into an envelope).
Turn the dough 90 degrees and roll out to a long rectangle. Repeat the and folding, then wrap the pastry in cling film and chill for 15 minutes.
Remove the skins from the sausages and black pudding. Crumble the black pudding into the sausage meat and stir to combine.
Lay a large piece of cling film on the work surface. Place the Parma ham on the cling film, over-lapping at the edges to form a neat rectangle 28x20cm/11x8in.
Spread the tomato and mushroom mixture over the ham leaving a 1cm/½in border. Spread the sausage meat on top of this. Then line up the boiled eggs lengthwise down the centre of the rectangle.
Using the cling film to help, roll into a tight sausage, keeping the eggs in a line and in the middle of the sausage meat. Twist the ends of the cling film to seal and and chill in the fridge for 15 minutes.
Take the pastry from the fridge with a remaining quarter of the butter. Repeat the process of dotting with butter, rolling the pastry, folding and chill again for a further 15 minutes.
Unwrap the sausage and place it on a lightly oiled baking tray. Bake in the oven for 15-20 minutes. Remove the sausage from the oven and cool completely.
Take the pastry from the fridge and repeat the rolling, dotting and folding for the final time. Chill until ready to use.
When the sausage is cold, roll out the pastry thinly. Cut into a neat rectangle around 38x25cm/15x10in and reserve the trimmed edges for decorating.
Place the sausage in the middle of the pastry and cut off small squares at each corner to avoid too much pastry at the ends.
Fold the pastry at each end up and over the sausage, then wrap the long sides of the pastry up and around. Brush the overlapping edges with water and gently press the pastry to seal.
Place the Wellington on a baking tray seam side down and brush all over with beaten egg.
Cut thin strips from the pastry trimmings and make a lattice pattern over the Wellington. Brush the decoration with the beaten egg and bake for 45 minutes. Check the Wellington after 30 minutes and cover with foil if it is browning too much.
Allow to rest for 10 minutes before serving.