Blueberry and Lemon Surprise Pudding

This is a variation of the well-known 'surprise' pudding, where the mixture goes into the dish as a, frankly, curdled mess and emerges from the oven with a fluffy lemon sponge on the top and a lemony saucy layer below.  I added some frozen blueberries, an idea I got from the splendid cook, Lady Claire MacDonald, who runs the Kinloch Lodge Hotel in the south of Skye - I will stay there one day......

Anyway, to the recipe, and forgive me but it is in Imperial measures, as befits its source: in a bowl, beat 3oz of very soft butter with 6oz of unrefined caster sugar - it won't go fluffy.  Add the zest from 3 organic unwaxed lemons and 3 egg yolks.  Then stir in 3oz flour, the juice from the lemons and 7fl oz whole milk.  It will look very curdled!  Now, beat the three egg whites until the stiff peak stage and fold through the mixture.  At this stage, you will be panicking and thinking it is ruined.  Trust me, it isn't.  If liked, stir through 4oz of blueberries.  Tip into a buttered baking dish, not too shallow.  Bake at 160C fan oven (180C non-fan) for 45 minutes or so until the sponge top is a nice deep golden.  Dish up, making sure you get a spoonful of the saucy bottom layer too.  This is just as nice cold the next day, although the sauce will set into a tangy 'curd' custard.