Try Ashton Artisan´s blend. Not quite the same but, as someone said in this forum, it "scratches the same itches" for me. Waiting to try Robert McConell Covent Garden when I can get it. It is supposed to be inspired in NIghtcap. I am sure the Dunhill blends will come back under other names. Too popular to go out of the market.
There is a thread on here that has kind of slipped through the holes about C&D's Mountain Camp. Apparently very close to Nightcap and perhaps even better. I have a few tins of Nightcap but might have to pick up some of this Mountain Camp and give it a try.
I still have one sealed tin of Nighcap and am inclined to try C&D's Mountain Camp, and maybe Poplar Camp (even stronger). Nothing is going to be Nightcap, but there are blends that fill that gap, that in fact that some might prefer, or come to prefer. You have to resist the tendency to adore each blend that is discontinued. Right now there is an overwhelming variety of blends of all kinds and we sound like whiney rich kids when we lament on blends we can't get.
The whiney rich kid would tell his analyst his mother wouldn't buy him any more Nightcap and it ruined his life. Heck, I long for some nostalgia trips myself. My dad used to buy a hometown Chicago beer, probably the cheapest, in big quart bottles, called Meister Brau. (Master Brew.) It was the first beer I ever sipped, and he was easygoing about my having some from the refrigerator. I can still taste it and no artisan brew comes anywhere close. Its hops were not just tasty but had a special twang.