Yeah i would of thought it would be the thin blue wire as i said, thats for the battery light.

Yeah a diesel one is more powerful, but in that case you might need to uprate your battery especially if its getting on a bit, mine has got a starter i saved off a MK5 1.4 escort i had, the original ford one, but starts it perfectly, shouldnt really be any need to go getting bigger starters to be honest.