For the bun dough, dissolve the yeast in 200ml/7fl oz of the lukewarm water.
Sieve the flour, sugar and salt into a large bowl. Rub in the butter.
Add the whisked eggs and the yeast mixture and enough additional water to make fairly soft dough.
Cover and leave to rest for 10 minutes.
Turn out onto a floured surface. Knead well, about 5-10 minutes, until the dough becomes firm and springy.
Put the dough into an oiled bowl, cover with cling film and leave to rise until it doubles in size – up to an hour.
Turn out onto surface and knead for 2-3 minutes. Leave to rest for five minutes.
For the poppy seed filling, grind the seeds in a coffee or nut grinder.
Heat the milk, sugar, orange zest, ground poppy seeds and raisins in a small saucepan over medium heat. Cook gently until the seeds absorb the milk, and the mixture thickens to spreadable consistency - about 15 minutes.
Add the lemon juice, brandy, orange liqueur and butter and cook for another two minutes.
Finally, add the vanilla and stir to combine. Remove from heat and turn out onto a medium-sized tray to speed up cooling. You can also be put in the fridge if necessary.
While the dough is proving, grease and sugar a 13 x 10in/33 x 25cm, deep sided tin.
Preheat oven to 200C/400F/Gas 6.
Roll out the dough to a 26 x 40cm/10½ x 16in rectangle. Brush the surface with melted butter and spread the poppy seed filling evenly over this, leaving a 2.5cm/1in border all the way round. Roll the long side of the dough forward into a fairly tight roll.
Moisten the farthest edge of the dough with water, and press the roll of dough onto it to form a seal.
Brush the log with melted butter.
Cut off about 2.5cm/1in at either end for neatness, and slice the log into 12 equal portions.
Place each bun cut-side down in the prepared tin, leaving a small gap between each one to allow for expansion.
Brush each bun with egg wash, cover with tea towel and set aside to prove for 30 minutes.
When the buns have proved, bake in the pre-heated oven, on the middle shelf, for 18-20 minutes.
Meanwhile, place the ingredients for the bun wash together in a pan and bring up to the boil. Boil for 3-4 minutes to create syrup. Keep hot.
Remove the Chelsea buns from the oven and brush immediately with the bun wash.
After five minutes, turn the buns out onto a cooling rack and separate each one. Allow the buns to cool a little then serve.