dodge s56 horsebox?

Really strange..... turned the ignition key half way as usual to warm up before start then full turn... all I got was CLICK.... had this before once and it was the starter motor... Jump started the lorry from car .... works straight away... Battery replaced last May.... then a week later tried it Friday pm ... started first time. Went to use it Saturday the same thing.... Nothing just CLICK... again jump started it and ran fine!!! what that can be about??? anyone any bright ideas??? much appreciated