Dispute after flooding

My property flooded due to a blockage in a toilet. The plumber said the ball-valve was stuck and therefore caused water to spill over the bowl. The blockage was cleared and the valve was repaired. The work description was as follows: Called to the property to find the ball valve constantly overflowing and blockage in WC. Cleared blockage and drained off cold, removed and replaced with new. Filled system and tested, all ok. Even though the tennant was not living in the house, 2 days later, the plumber was called out again by the letting agents and found another supposed blockage. The job description was as follows: Called to property to find blockage in underground pipes. Rodded drains externally but couldn't reach the blockage. Remove WC and rodded internally to clear the blockage. Re-installed WC including new seal. Filled system and tested, all ok. As this was deemed as emergencies, the invoiced price is £604. Asking for a second oppinion since I believe the letting agent is pocketing from these fees.