2008 ML 500/550 slow to crank

The domestic goddess's ML500 (Australia) has 110,000km, she has had it for 3 years from 80,000km

Great car, no major issues except I replaced the rear airmatic bags at about 100,000km when one developed a leak

The engine has generally been slow to crank since she got it. It's as if the battery was going flat

It cranks slowly for a few revolutions then spins normally and the car always starts after, maybe, a few seconds

Sometimes those first few cranks are so slow that you think the battery is just not going to turn it over then away it goes

We replaced the battery after about the first year but it made no difference

It failed to start for the first time a few days ago. It just cranked slowly and eventually cranked no more.

It eventually fired up "normally" about 20 minutes later after she let it sit a while. She wasn't very happy

It's done it's usual thing for the last few days

I have hooked the car up to my Star Diagnosis setup on a few occasions but no relevant faults are found

I am thinking that it might be the starter itself but it works perfectly after the first few slow cranks and if the engine is switched off and then started after a minute or so it works perfectly if you count one slow crank every time.

I've searched the various forums but there doesn't appear to be anyone out there who has experienced a similar problem

Does anyone have any clue as to what might be the cause of the problem?

Thanks

Peter