Gratin of Penne with Leeks and Bacon

I love macaroni cheese - it is such a warming thing to eat on a cold night like tonight, and Delia's take on it includes leeks and a little bacon for savoury yumminess.  Here is Delia's recipe - I didn't use the cream, as I thought it rich enough, but by all means go ahead and use it if your arteries are younger than mine.