Tear away coarse midribs and thoroughly wash spinach in cold water. Pack wet spinach into a saucepan, cover with a pan lid and cook over moderate heat for 10 minutes until spinach is tender. (The water clinging to the leaves will provide sufficient cooking liquid.) Drain well by pressing the spinach in a colander, then finely chop or pass through a food mill.
Melt the butter in a saucepan over low heat. Stir in the flour and cook gently for 1 minute. Gradually stir in 1/z pint (300ml) of the milk. Bring to the boil, stirring well all the time to make a smooth sauce. Simmer for 2-3 minutes to thicken. Draw off the heat. Stir in 3 oz (75g) grated cheese and season with salt and pepper. Stir half the sauce into the spinach puree. Spoon a little spinach filling down the centre of each prepared pancake and roll up carefully. Arrange pancakes in a greased ovenproof dish.
Add the remaining milk to the rest of the sauce and bring to the boil. Check seasoning and pour the sauce over the spinach pancakes. Sprinkle with the remaining grated cheese and heat through in an oven heated to 375°F (190°C) or Gas no. 5 for 20-30 minutes until bubbling hot and brown.