There’s one that I was proper obsessed with the atmosphere of for a while but couldn’t remember the name. Watched it recently and really enjoyed it (though I’ve forgotten the name again – I’ll do a quick Google)
One quick Google later: The Ghost of St. Michaels!
Good memory of being at my nan and granddad’s when the last episode was on. Nan came in the room and said “You don’t normally watch Poirot,” and my granddad replies, “It’s the last one and I want to see if he dies.”