Management company charging a late payment fee

Hello

looking for help/advice, apologies if this is longwinded. Have been renting out a house for almost 10 yrs. Same Auctionering Company/Estate Agent collecting the management company fees throughout, and I've always paid the managment fee on time and in full, except for two years when I received no invoice from the management company. When they contacted me with a large bill I paid it imediately only to have them send me another bill for a late payment charge for those two years when they didn't invoice me. Last year before the managment company ie the home owners AGM I wrote to them saying I didn't see why I should have a late fee applied as the mistake was on their part. No reply so I assumed all was well. Got another invoice for this year again with the late payment fee still on it. I've paid this yrs management fee and I'm getting nowhere with them re the late payment fee. I'm asking them for some goodwill but all I get is we have to talk to the manangement company ie the home owners where do I go from here. thank you