I read as far as "It consists of a biscuit base, an apple and solid custard-ish (but not quite custard filling) with a soft caramel topping", then dribbled so much I had abandon the rest of your post to google the recipe.....only I can't find it!

Lots of " 'Caramel Apple Granny' (aka Caramel Apple Betty, aka Caramel Apple Cake) recipes but they are all variations on a theme. None are exactly as described by you - no wonder you described it as "elusive!"