Section 106 Variations

Trevwil

We are part of a new housing estate that has a management company looking after the green areas etc of the estate. Recently they asked us for additional payments to replace a small footbridge over a waterway just outside the boundary of the estate. After some back and forth it was agreed that fell into the section 106 and was for the council to replace/repair. Now the issues start arising the section 106 says that the balancing ponds and swales will be maintained by the council yet we are paying a management company to do them. I asked the council this and they said an informal discussion/agreement had been made with the council and developer that the management company would be responsible. I asked the council for an up to date S106 but got told that there was not one and no deeds of Variation to the existing S106 only informal agreements. Firstly is that legal? as all information says any variations should have a deed DOV to be legal. If it is a case that the S106 legally still stands without change what course of action would I need to take?