Preheat the oven to 200 degrees C. Put the sausages on a baking tray and in the oven for 15 minutes until golden and cooked through. Alternatively, fry in a pan until golden.
Preheat the grill to high.
In a saucepan, melt the butter over a medium heat. When bubbling, stir in the flour and cook for 2 minutes.
Gradually add the beer, stirring all the time, until fully combined.
Stir in the cheese, mustard powder and Worcestershire sauce. Cook for 3 more minutes, stirring regularly, then remove from the heat. This is your rarebit.
Put 2 slices of sourdough onto a baking tray. Spread the rarebit over the top of the two slices. Place under the grill for 4-5 minutes until bubbling.
Remove from the grill. Cut the sausages in half lengthways. Place 4 sausage halves on top of each piece. Top with the remaining two slices of sourdough.