How to remove remnants of a failed iTunes movie download?

Question

Hi, please let me summarize my recent experience with an iTunes movie rental. At this point, I mainly want to know how to regain the space on my iPad. The whole disgrace of why the download was slow, why iTunes on my mac did not know about the rental initiated on my iPad (so I had to buy it again), or the fact that after the failed download iOS seems to have left a huge blob on my iPad "forever" stands here to flag this issue for fellow users and Apple.
Let's start with the issue that would help me solve the situation as things stand now: Now I have the movie downloaded to my mac's iTunes, finally, but now I cannot sync it to the iPad, as the latter has no free space --- as the failed download of the movie there left there a large "file", which I cannot remove. The file is not seen by the iPad's iTunes Store or Movies apps (there I deleted what I could), I closed both apps specifically, I restarted and synced the iPad multiple times, but the free space is not recovered.
The background is that our Friday movie night ended up being a 4-hour ordeal attempting to have a fraction of the movie downloaded to start watching, and even worse, the download did not finish (or even continue!) while we started watching, but failed, and failed miserably, losing me even more time now. I started renting the movie one more time, then on my mac, hoping that the download will be faster there, but it wasn't, and now that I have the movie, I still cannot play it on my $500 "resolutionary display"...
Not even to mention that iTunes on my mac did not know about my rental on the iPad. So instead of the promised "seamless" continuiation of the movie on the MacBook (which I hoped will have a[n even] better connection to the iTunes Store, partly because of its better antenna --- it did not), I had to pay up again, restart the download, and still see it only trickling down.
The whole problem started with the download speed being unacceptable. I checked my downstream bandwidth, which was 16 MB/s. Then offering me to download the movie in 5 hours or so is more than disappointing.
Funnily enough, in the end now my mac's iTunes realized that there were a purchase on my iPad, and restarted the "original" download. So now I have two 4.5 GB copies of Tinker, Tailor, Soldier, Spy on my mac! What a waste of space and bandwidth. But of course, iTunes keep track of me having started "watching" one of them, so that will soon expire. Pfff.
What a disgrace this is. Google Play, Amazon Instant, Netflix must be gleaming. I only tried iTunes for the 1080p experience on my expensive new iPad.

Answer 1

And there was more. I had to re-rent the movie on Google Play and finish it there, because the iPad won't play its downloaded fragment, nor did my mac's iTunes, which already downloaded two copies of "Happy" and three copies of the twice-rented Tinker, Tailor..., would show ANY of that under Rentals nor allow copying ANY to the iPad during syncing (though the movies at least showed up there, see the note three steps above this).
And iTunes would still start downloading another copy of both movies whenever it checks for "unfinished downloads". I think my HDD will be fully of copies of the same movies soon.
And the iPad still have a huge chunk of "Tinker, Tailor..." lingering on somewhere in the non-transparent file system...
At least the iTunes support guys ("Express" Lane after 24 hours, though over a weekend) were nice in their email, and I got my money back. Now I only need my time, sanity and storage space back.