i'm assuming its a 2.5 transit diesel engine if so-check engine earth lead and battery earth lead for clean secure connections,also check main lead to starter,especially where lug on starter end is crimped [have had plenty of these with poor connection here ] may be worth soldering this lug where crimped on lead ,if you have access to solder iron .make sure battery is correct cca rating ,if all ok possibly faulty starter.
k
December 2010