Change in management expenses terms from new management company query

I live in a flat so have been responsible for paying a percentage of management company expenditure for many years - when that company got involved with the property, they saw no need for a typical annual management fee. If and when works were needed, they would simply increase the amount to pay - so as such, the management fee was based on % of expenditure. I also was paying ground rent to them as part of the conditions of my lease.

Two years ago, my main contact at that company informed me that there would be a new company involved with the management of the property, which he was involved with. He assured me that terms of payments would not change, obviously something which I should asked for in writing. By this point, the ground floor had been converted from a shop to there being two flats - it was not something that particularly bothered me, thinking that it would also add value to my flat with it now not being above a shop.

I have not received any demands for my share of buildings insurance or ground rent for two years, despite chasing it up many times (I did know that buildings insurance was active on the property though). After chasing it up again before Xmas, I finally received an invoice. This also has an annual management fee charge, which has surprised me. Having queried this, pointing out that I was told that there would be no change from previous arrangements, I have simply been told that this company pass on all expenses at cost and charge a flat fee annually.

Is this something which I can legally challenge? In all fairness, the annual fee is not much - but there has not exactly been active management of the property.