Are you talking about a management company or managing agents? Because if it's agents, check the terms of the contract with them and try to dismiss them or indeed if they have failed to fulfill the terms of their contract, they should also be threatened with action.
Usually in developements there is a clause in the planning permission that the builder has to enter an insurance bond for a fairly substantial sum which is a guarantee for the satisfactory completion of the estate- so check this out, this bond should be renewed until the estate is complete. Your solicitor will have been given details of this bond when you purchased the house. Inform the council of the situation and they can also take action, which may mean you wont have to.
It looks like you're in a good position as this insurance bond is probably in place, and you have the additional protection of homebond. I cant say whether the threat of legal action will suffice, every situation and every developer is different. The most important thing to do is to get appropriate engineering and legal advice and know where the legal and engineering costs are going to come from- i.e. will you be able to recoup them.