Melt the butter in a saucepan and stir in the flour. Combine to make a roux, then, with the pan off the heat, slowly add the milk a little at a time, stirring well after each addition with a wooden spoon and taking care to keep the mixture smooth.
When all the milk has been added, slowly add the cider in the same way, stirring well.
Put the pan back on the heat and bring to the boil, stirring all the time. Turn the heat down, add the mustard and season well. Taste to check that you have enough mustard (though it shouldn’t overpower).
Stir in the cheese and double cream, if using, keeping a bit of the cheese back to sprinkle on top. Taste and season accordingly.
Cook the macaroni in plenty of boiling, lightly salted water for a couple of minutes less than it says on the packet. Drain.
Combine the macaroni with the sauce, pour into a gratin dish and sprinkle the remaining cheese over the top.
Bake in the oven until crispy, bubbling and golden brown (this should take about 20 minutes).