How can i fix my x reg ford transit pickup 2.4L TD starting problem?

Hi, i've bought an x reg ford transit pickup 2.4L TD off ebay and only had it a week. It was fine when it was delivered, but since then i turn the key and just get a click. It has turned over acouple of times on the warmer mornings, but really slowly, i would be so greatfull if someone could help, thanks.