I suspect the plumber hiked his prices anyway as it’s an insurance claim. Remember they do have travelling time, time to get materials and supplies and have to run their vehicles, maintain tools and licenses, when you consider how long they were on site. Same for any trade.
It hasn’t been stated by yourself whether the leak was as Susan13 has correctly outlined.
Clearly, if the leak is in the building fabric, e.g., wall, floor or ceiling, then it’s the buildings insurance covered by the Management Company. Also, you haven’t said whether the management company is a group of residents or a professional agent.
In many deeds / leases the managing agent / company/ committee has the right to enter the flat and inspect works, If they believe the works are not acceptable then they have the right to demand these are fixed.
Then we have the word “acceptable”, what is acceptable to one may not be to another. The deeds or lease could state that the works must be done in a ‘workmanship manner’, that doesn’t mean profession but it does mean not by a cowboy.
When you obtained your property and all AGMs or changes since, you should have been updated and attended any meetings where any impact of decisions made could affect you. I highly recommend you do in the future if you have not in the past. Also, if you believe decisions made are totally unreasonable and unacceptable you need to write to the Managing Agent (M.A). Whether you win your case or not it needs to be put on record.
If you haven’t received the ‘process of claiming’ from the M.A then they cannot threaten you. Likewise, if your claim is significant and above the excess, they have no right to refuse to process your claim. Normally a process of claiming ensures, as I have said, a central point of managing these and keeping down premiums.
You need to read your lease (several times), plus any common agreed rules and regulations and if updated. Many of the latter are to help run the site to the common good, but not always legally enforceable. If you feel you do not have any of these or have not been updated, this will aide your corner but you legally must be provided with copies.
Also, if you want a clean slate get the plumber back to finish the job to a good standard and pay them. Before you pay you must obtain a copy of their liability insurance in case things later go south. That way you cannot be accused of ‘harvesting’ a claim to your benefit.
All works must be to a good standard and to regulations.