Just anecdotally (a senior associate at Slaughters told me this) I've heard partners have significant influence if they really like or dislike a candidate.
A lot of the big US firms are like this too. If a senior partner likes you, you're most likely in. No senior associate or member of recruitment can challenge them.
I also have a few friends who have relatives at the MC firms from which they got their offers. I would give them the benefit of the doubt and say that they secured their TCs on their own merit, but behind closed doors, they'd brag about how their dad or uncle ultimately got them the job.
I think it's just how it is and it's one of those things that most likely won't change.
At least these firms recruit the other 99% of us fairly.