Place the milk into a pan and bring to the boil.
In a separate pan, melt the butter, add the plain flour and cook over a low heat, stirring frequently, for two minutes.
Gradually whisk the warmed milk into the butter and flour mixture. Bring back to the boil, whisking continuously.
Whisk in the egg yolks and bring back to the boil before adding the mustard, Worcestershire sauce and ale. Add the grated cheese and whisk until smooth.
Preheat a grill to medium.
Spread one side of each piece of toast with the rarebit mixture. Place under the grill until golden-brown.
To serve, place onto serving plates.