Can I make ricotta using milk that's been kept in a cold fridge, in an unopened jug, but is a few days past the sell-by date? Thanks! ;o)

Just a guess: when you make yogurt you scald the milk as the first step, to kill bacteria that would inhibit or spoil the culturing process. I think the first step in the ricotta recipe serves that purpose.

You surely can, especially since it hasn't been opened, which means that no outside bacteria has been introduced into the jug. I wouldn't drink it straight but for ricotta it is fine. As an alternative to the creamy ricotta, if you don't want to risk wasting extra cream, you could do just a plain milk, lemon juice and salt batch, drain it well and then make yourself some baked ricotta, which can later be crumbled on salads.

The only time my ricotta ever fails is when any or all of my ingredients are a little old and they are open. It's only happened a couple of times, but I have had the recipe fail with older ingredients. Just FYI.

Milk is actually good for quite some time beyond the marked date.

Yes, unless it is obviously spoiled (a quick whiff will alert you!). In fact, even tangy milk is excellent for cheese making, it's not until you get to the seriously funky chunky stage that red flags wave.