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You would raise it with the managing agents and say 'I don't think you've earned your fees for XYZ reason and I consider ABC to be payable'. When they disagree you can tell them you'll be applying to Tribunal for a determination unless you can reach an arrangement.
Going to Tribunal is a lottery - if you're reasonable you'll have more chance of winning, but some judges are pro-Leaseholders while some are pro-Freeholders. If you get the latter, you could get fucked. So use it as a threat unless you really have to engage.
Ask me how I know this. I'm currently fighting a Tribunal appeal which looks like it's going all the way to the High Court. Some Freeholders are cunts.
Who would judge whether or not they have been useless?