If you are getting repeated cracking, you might inquire about replacing the cartridge for the sunroof (I think that's the term that my servicewriter used). We did four arm & motor replacements on our Freelander (the arms would break or bend, causing a jam) and the motor would fry out. It turns out that the cassette (the area that the arms fit into) was defective or out of proper alignment; LR only warranties replacement of the arms & motor, not the actual cassette, because the cassette is a fair bit more expensive, thus the repeated replacements.
When I pointed out that we were burning through them every 6 months or so and that we'd just keep coming back (dealer replaced items are warrantied for 1 year from date of replacement) until we sold the vehicle, they relented and replaced the entire cartridge. We didn't have any difficulty after this point.
Obviously we're talking about different vehicles, but the warranty rules are probably the same across models within LR....