Simarc unprofessional & bullying tactics

I own a flat in a London development where very unfortunately Simarc not only manages rent collection but it turns out is a subsidiary of the Landlord Company (Wallace) - I wonder is this legal??
Simarc has charged me extortionate fees for missing ground rent payment for last year and the current year (my bad but I never received request of payment!!).
They have replied to my emails but they have not addressed any of my points and have not provided explanation or breakdown of the admin charges, despite my repeated requests. I find their approaxh blackmailing and bullying.
I read the previous reviews and I understand I am one of many.
But has anybody in London taken them to court / followed a procedure and managed to get the admin charges waived as well as making them pay for any legal expenses incurred by the flat owner? I could use some advice here.
The joke is that at the back of the invoice it is stated that "the admin charges are payable only to the extent that the amount is reasonable" !!!!