I bought it on NTSC, so didn't bother with it for my PAL machine, and have had no problems but a few people I know have had problems with the PAL version.
Don't worry, it is what you said earlier, the lenses must be shite for dual layer. The reliability of Nintendo's HW is usually excellent, but this time they've settled for poor drives when it comes to dual layer stuff, unfortunately. If I wasn't so particular about keeping dust away from Wii, because of the reported problems, I could've very easily had issues.
If you haven't modded your machine try contacting Nintendo UK, because they have said they will sort it out, free. And try and get some compensation, like some free Wii Points, for the inconvenience.
http://www.nintendo.co.uk/NOE/en_GB/news/2008/super_smash_bros_brawl_game_disc_information_8780.html