Nick while not great, was no worse than some Marius's ive seen (e.g. Gareth Gates)
I really didn't like Matt Lucas as Thenardier, he did all these unnecessary ad-libs that I just found annoying and I found his voice similar to Chris Moyle's as Pilate, thin and a bit strained with no depth and sounding an awful lot like they needed to clear their throats or something (which I put down to not having the range for the role). Matt also did a lot of random squeaks on words that got on my nerves and he and Chris were allowed to just play themselves in a funny costume, both of them seemed more concerned about getting a laugh out of the audience than getting into the depth of the character. You're supposed to walk out of Les Miserables hating Thenardier because he's despicable, amoral and selfish, not because you were sick of seeing him onstage. I couldn't see an Eponine that turned out as fierce as Samantha's was growing up under the influence of Matt's bumbling Thenardier.
And with Mel C, I just can't see her as an Ellen. Vocally, she was very good in JCS (though a bit breathy) but Ellen goes higher than Mary, and she'll be 40 when the show opens and Ellen's supposed to be in her early 20s.