Hi. Had same problem with my S type. The battery was low in charge, a new one sorted problem. To test put your battery on charge then try. If all is ok you need new battery. Because of the length of cable from the battery to starter motor there is quite a drop in power available, especially if the battery is showing its age. Don't waste your money on expensive dealer tests, if need be find a good local old fashioned garage with a decent sparky. Hope this helps, and buy a heavy duty battery it helps a lot, and needn't cost the earth if you look around.