Difficult. You should have given a weeks notice. It seems you did not. However it depends how nasty you want to get. Threaten any emplyer with taking them to a tribunal and they usually pay up, why? Because tribunals side with employees and employers have to pay solicitors to defend them. Personally though I would write to them tell them you are disappointed, that you showed them loyalty and this is demonstrated by the fact that you know the male partner is having an affair (do not mention the female partner) yet you kept it quiet. Then tell them you are considering all options.