I thought the Alhambra was more impressive. But then I have a castle fetish :).
Aha! Now, with this inside information, I begin to see my way towards a plan.
All you do, right, is pay a visit to the gallery, okay? Then, while your accomplices distract the guards (a faked heart attack, maybe), you whip out the hammer and chisel you have secreted about your person, and knock a segment off the nearest marble. Make it fairly small, so they don’t notice, and then you can simply saunter out of the museum with it in your pocket, pop it into a Jiffy® bag, and post it off to a confederate in Greece. Repeat as often as necessary (I suppose you’d need to vary your MO a little – different times of day, different accomplices and distractions, and so on), but there’s no heavy lifting involved, and all the guy in Greece has to do is glue the bits together as they arrive.
I really don’t see what could go wrong.
You fool! That’s just what they’ll be expecting!
Aside from the fact that I think the Marbles belong safely in the British Museum, security would think I’d gone mental if they saw me with accomplices, let alone doing anything like that!* 
They’re all sound people, the warders; I’ve got to know quite a number of them socially over the years from visits and as well as my work P & E, and am looking forward to seeing them again this August.
*I admit, I was and am a terror when I had/have my security clearance as people will not keep their hands off ‘my stuff.’ 
Well, now, this is my fault for being unclear: when I wrote “you” I didn’t mean you, I meant a more general “you”, ie anyone contemplating this particular heist – astro, possibly.
No offence, but you wouldn’t be my first choice for the job in any case. For one thing, you’re already a known face to the guards, which is going to make the whole “slipping in and out anonymously” thing a bit tricky. For another, the fact that you haven’t grasped that you’re not supposed to let the guards see you with the accomplices suggests to me that people who work in museums probably have a poor understanding of the subtleties of the criminal mind.
Which is pretty much what I’m counting on, if I’m honest.
So… were they stolen or not?
Fortunately, I don’t work in a museum (my gig at the BM was a sabbatical contract); I’m a university professor. 
My work there now is research-orientated.
As far as pinching things from museums, in my experience, some of the best criminals have been people well-known and in positions of trust – because they are the ones you’d least suspect, and it has come as a shock when some of them have been discovered (my apologies for being so cryptic, but the one case I’m best familiar with is still being sorted, and I’m not sure how to describe it without giving away too many details.)
As for the warders – while they are brilliant at their jobs, the museum can get so busy, especially in summer, that sometimes things go awry – Banksy managed to plant a fake ‘artifact’ recently in one of the galleries that was there for about three days before anyone twigged.
I am sorry I misunderstood your plan – what I had meant was your particular plan would not have been suited for me. 
The acquisition of the Marbles is a subject of argument, but they were purchasd by Lord Elgin (the Museum still has the receipt); as I understand it, the gist of the argument of the Greek government is that they (the Marbles) were purchased under complicated circumstances which they (the Greeks) claim is a valid argument for the Marbles’ return to Athens. Someone more familiar with the case can undoubtedly give you a much better explanation.
In fact, I did initially consider an inside job. The old “taking them away for cleaning” ploy is a classic, but I thought that in the case of the Elgin Marbles it might raise a red flag. 
Very sensible – I always do the same myself. Means you can take them back if you find you’ve bought the wrong size, or whatever.
This is why I love the British Museum- the whole place is full of all sorts of weird and wonderful things [del]nicked by British soldiers[/del] rescued from all manner of exotic places and put on display so we can marvel [del]at the effectiveness of Gunboat Diplomacy[/del] at the rich and colourful history of our world [del]of which the British used to rule a sizeable percentage, but don’t anymore[/del]. 
Seriously though, I love the British Museum- it’s one of my favourite places, along with the Imperial War Museum- and most of the rest of London, for that matter.
Purely non-constructive point: how much longer would Greece have been Ottoman without the British (well, overwrought British poets like Byron, and British soldiers still laid-off since Waterloo)?
If we can always use the “If it wasn’t for us you’d be eating knockwurst in your toad-in-the-hole” line, why begrudge the Bits “If it wasn’t for us you’d be drinking cardamom in your coffee?”
Given that the Ottoman Empire collapsed shortly after the end of WWI, I would say that- assuming the British hadn’t been cruising around the Mediterranean for much of the 19th century and carting anything of interest back home (not to mention getting involved in various local conflicts along the way)- Greece would have ceased to be under the rule of the Ottoman Empire around July 24th, 1923.
We prefer to think of them as ‘borrowed with extreme prejudice.’