[Tim C]

Could be any or a combination of all the point mentioned. Duff battery, lazy starter or dodgey connection.

If you are jumping it from another running car, the 13-14 volts that is produced from a charging alternator can be enough to overcome the latter 2 reasons. Similarly, a weakness in a battery will be hightlighted by the latter.

A process of elimination I'm afraid. Although as it restarts after a short period of time it would point towards the battery. Having had a quick recharge from the alternator it's OK , but it decays away after a while. You could try leaving it on a charger and then see what happens.

As well as the battery and earth strap connections it could be the starter motor connection. Or even the bare conductor that links from the solenoid to the starter motor itself, mine eventually vapourised on the quattro. As it gets gradually thinner through corrosion, it becomes too thin to carry the current and then acts like a fuse wire .