Simon's Grandma used to make this every year - the family loves it.
Here is her hand-written recipe, transcribed below.
Her cooking was delightfully imprecise, and all the better for it!
Method:
Mix plain flour to a paste with cold milk.
Boil the rest of the milk, add some sugar & a nob of butter.
Then mix as any white sauce, except that there should be more flour in it.
Stir in nutmeg last, and just taste until it appears right.