A man keeps trying to kill his wife, but a ghost keeps stopping him. In the end the husband is in the bedroom with his mistress and the ghost kicks out an ember from the fireplace to start a fire.
Pretty sure this is The House in Marsh Road (1960).
Jean Linton (Dainton) has inherited £1000 and a country house, Four Winds, from her late aunt. She and her husband David (Wright), an alcoholic would-be novelist, move there, and are told by housekeeper Mrs. O'Brien (Anita Sharp-Bolster) that the home is already inhabited - by a poltergeist she has named 'Patrick', after her late husband. Patrick makes his presence known to Jean by moving furniture about, breaking mirrors, etc.
Unhappy living in the country, David meets an estate agent at the local pub, who offers him £6000 for the house and land. David, however, can do nothing as they are in Jean's name and he is unable to convince her to sell. He hires local sexpot Valerie Stockley (Dorne) - she calls herself 'Mrs.' but her true marital status is questionable - as his typist and they begin an affair. Valerie tells David that once her divorce comes through, she'll be free to marry him, as he has proposed. But when she learns that Jean controls the property and the money, she angrily suggests that David kill Jean so that he can inherit.
Patrick becomes quite protective of Jean after she discovers the affair. David makes attempts on Jean's life - trying to push her down the lift shaft in the house, dissolving an overdose of sleeping pills in her glass of hot milk - but each time, Patrick intervenes. He slams shut the lift gate before Jean can fall and causes a loud alarm to ring just as Jean is about to drink the doctored milk, which makes her realise that there is something wrong with it. Jean consults a solicitor in London about a divorce, but he tells her that her argument that a poltergeist is the only thing preventing David from murdering her 'won't hold water' in court.
With Jean in London, David is free to have Valerie spend the night with him. Patrick takes the opportunity to punish them. As they're lying in bed, Patrick locks them in the bedroom and sets Four Winds aflame. When Jean returns later that night, having been driven home by her close friend and confidant Richard Foster (Derek Aylward), she learns that both David and Valerie have perished in the fire, trapped behind the bedroom's barred windows.