How do I cook a cottage pie from chilled?
It's very difficult to give an answer, as the size and shape of the casserole can greatly affect the reheating time.

I've found to get the best results, I start the casserole covered in a cold oven (put it in the oven, then turn the heat on), and cook it at low heat (300°F / 150°C) until it's heated through.

Once it's up to temperature, I uncover the dish and turn the oven to broil (called 'grill' in some areas ... when only the top element is on at very high heat), and brown the top.

If you need to, once the dish is warmed through, you can turn the oven down to its lowest setting and hold it there until you're near time to serve it.  (it only takes a few minutes to brown the top, especially if you adjust the oven rack so the casserole is an inch or so (2.5cm) from the heating element)

I'd probably give myself an hour minimum to heat it, preferably 1.5 hours, and 2 hours if it's an especially large dish.