Good advice that I concur with.
In addition I suggest you think through what long-term aim this developer is trying to achieve. It may be that he is seeking to force down the leasehold value of each of the properties on the site so that he can buyout the whole site, then seek planning consent for something very different on the whole area - you need to consider the bigger picture and it might involve this.
The second possible scenario is that he discovered that site ownership obliges him to maintain the common parts but that he isn't obliged to do it to a std and he is just trying to milk as much as he can for it, from the lease and rent obligations without spending his part of the obligation. Have you spoken to anyone on the leasehold part of the site to see how their lease terms are worded regarding grounds maintenance.
A pressure strategy (which may help with pushing the Local Authority twerps along also) could involve mustering as many of the residents together as a collective pressure group (yes, work I know, but needs must) and seeking the involvement of the local press in your campaign of naming and shaming.