Loose earth connection, loose connector onto starter, loose battery connection. I had the latter with similar symptoms. It was only loose to the extent that it just turned on the battery terminal when i heaved on it to remove it and change the battery. I cleaned up the terminal which was corroded, greased it with vaseline and replaced the clamp which was loose due to being cracked, and it lasted for another two years. Kwikfit had told me that the battery was dying but they had only attached the tester to the loose clamp, not the battery itself, so look out for that. I believe the live cable to the starter comes through a connector block on the wing in the engine bay and that can be the source of a poor connection but not exactly sure where it is - would have to trace the cable.