Or it would be if it would move. Sometimes, the starter whirrs enthusiastically but having been unrewarded by either a spark or a supply of petrol, it loses interest and fades to an asthmatic wheeze.

Other times, it just clicks - signifying that, for some reason, the battery has forgotten what it is, and is hanging around in the engine bay imagining that it might be a hammer, or a fighter plane.

I therefore called an expert on the 600 and asked him to check out the points. That made me sound like I knew what I was talking about. I also asked him to check the carburettors. That made me look like I didn't, because the 600 is fuel-injected. Anyway, since he didn't need to check the carbs, he had some spare time, so I asked him to give the car a once-over to make sure it's mechanically sound.

In my mind, I knew it would be. You can always tell, just from looking at an old car whether it's been cared for, or whether the undersides are being held together by friction and bits of dried-up fox that it's run over. Mine was kosher. I knew it.

And I was right. The expert has had a chance to plug it into various machines and agrees that while the bodywork is a bit pitted here and there, the mechanical components are in good order. Apart from one or two little issues, which he has detailed in a short, 800-page document.

Let's kick off with the brakes. They are good except the fluid condition is low, the offside front disc is marked and will judder, the nearside front disc is smeared with oil and may need a new caliper, which costs £3,000.

The front pads are worn and will need replacing at around £500 a pop, both rear brake discs are rusty and marked, the offside rear anti-dive pivot is worn, the nearside pivots have slight play and the nearside rear anti-dive gaiter is perished and split. This means I won't be stopping in a hurry. But it doesn't matter because, chances are, I won't be going anywhere anyway.

The list of what's wrong under the bonnet is so long, it's easier to list what's good; the compression. Everything else is split, worn, broken, bent, leaking, flat, incorrectly fitted, backwards, upside down, rusty, empty or loose.

I called the man to talk through the issues and as I sat, for several days, listening to the tale of woe, I wished, for the first time in my life, I was James May. Someone who understands what's broken. Instead, I just made a selection of what I thought might be appropriate noises at various intervals.

Then we got to the V-belts. There are seven in a 600 and, combined, they take 50 of the engine's 300bhp. And guess what? They are not loose, leaking, broken or empty. But they are noisy. "Well," I said, "if they are being noisy and using all my power, let's just take them off."

This is not possible it seems, as without them the car will either not run, or will run briefly before exploding. I think the man on the other end of the phone thought I might be a lunatic.

Anyway, it seems that the propshaft coupling is good. The wiper arms may be bent, the rear bellows cracked and split, the front exhaust pipe bouncing on the road, the fuel tank dented and the fuel pump even noisier than the V-belts. But joy of joys. The propshaft coupling is good.

"Well, yes," said the man. "It's good. But the bolts holding it are incorrectly fitted". Of course they are. Wouldn't have expected anything less. And what makes this litany such a face-reddening, bowel-loosening experience is that this car comes from when Merc didn't make its cars out of diamonds, because they were too weak. The exhaust system, for instance, is welded together. With myrrh probably.

That makes a replacement not only expensive to buy but also eye-wateringly expensive to fit. Then you have the four electric window switches. Take a guess? Go mad. Let your mind run free.

Nope. You're wrong. They are £5,000. Not including labour. Or VAT.

The thing is though, even if I buy a whole trolley full of electric window switches, and several spare exhaust pipes, the Grosser will still have cost me less than Johnny Rotarian's 540i. And when it's working, gliding from place to place on its air suspension, I tell you this: it will be the coolest car on the roads today.