I would also check the earth strap 1st, one should go from the battery to the chassis, then another should go from the engine to the chassis this one is bolted onto the starter motor itself, if after trying a jump lead from the - of the battery to the starter itself makes no difference, then I would remove the starter motor and strip if down, make sure it is,nt full of oil (wash it out with petrol) and dont forget to grease both front and rear bushes on the armature , Its not a difficult motor to remove.
good luck
Alfie Beard, January 2011
