Why would a relationship from half a century ago ruin a 45 year marriage?
Then I watched this film a second time and I noticed all of the subtle 'clues' that I had missed the first time I watched it.
For example, Kate and Geoff have a Dog as a replacement for Children. The dog is a GERMAN shepherd Dog: Geoff's girlfriend from all those years ago (who he calls 'My Katya') was German. Kate now realizes that Geoff did not want Children with HER because he only wanted to have children with 'his' Katya.
Then there is the fact that this couple have no photos of their time together. Surely a couple that have been wed for 45 years would have lots of photos taken together, wouldn't they? And yet, Geoff has photos and all kinds of mementos of Katya in the loft.
Kate also mentions that she has never had a birthday party. The clue here is very subtle. Geoff has never bothered to celebrate his wife's birthday with her. It is not important to him that the Woman he has shared 45 years with might perhaps like to celebrate her birthday with her husband.
Then there is the scene where Kate and Geoff begin to make love and Kate asks Geoff to open his eyes, and when he does, he is unable to carry on making love to Kate. The reason for this becomes clear: When he has made love to Kate in the past, he keeps his eyes shut because he is thinking of 'his Katya' and Kate realizes this.
Geoff considers going to Switzerland and trekking up a mountain to see the frozen body of 'his Katya' and yet Kate gently reprimands him that he has never even gone for a walk on the Norfolk Broads wit her.
Do you see what Kate finaly sees? Her husband of 45 years has been emotionally betrayong her for 45 years with his dead Fiancee. Every decision that Geoff has made has been with the thought of 'his Katya' in his mind.
At their anniversary party, it suddenly dawns on Geoff (45 years too late) that he has been very cruel to Kate. He has been cheating on her with a ghost. And he also realizes that he actually needs Kate, and that he does actually love her. Perhaps not in the deep and profound way that he loved Katya, but probably for the first time in their 45 year marriage, he has acknowledged just how much Kate really does mean to him. But it is too late. Kate can never forgive him.
This is a very sad and haunting film.