Sorry no recommendations, we barely used a tutor and not an 11 plus specialist. I know that some of the tutors who advetise great success rates offload students around Easter if they think they may not pass. We used CGP and atom. Although I liked atom I think it creates unnecessary stress by covering a wider syllabus. Probably works better if you start early, rather than if you start in May!
There is an active redbridge 11 plus Facebook group. You may get a recommendation there (though more likely someone self promoting). Some of the parents there advocate very intense study regimes, so treat with caution if that's not your thing.