For anything major, a meeting should be called and the matter put to a vote with the majority decision standing. The cost can then be shared between all the leaseholders, including those who voted against. If the process is properly followed, the charge can be enforced. It is sensible for a meeting to be held every year or so just for the managing agent to keep in touch with the leaseholders and, if the leaseholders are also shareholders, an annual general meeting is a legal requirement. It is not necessary for most expenses to be approved by a meeting but it is necessary for anything major.
If you have already paid your share of the work that was done (even if it wasn't properly approved by leaseholders), you should write to say that you dispute any part of future bills that relate to the work. You should also say that you dispute the related tribunal fees as such fees were incurred only in relation to other leaseholders. Disputing charges is always likely to result in threats of court action and so on so you need to accept this if you are not going to pay.
Incidentally, if you do have a meeting, you should ask the managing agent the specific question whether they recieve commission or other payments relating to costs incurred carrying out the management. It is not unusual for some insurance brokers to charge, say, £2000 for building insurance and then give £1000 straight back to the agent when a similar policy could be bought for £800 elsewhere.