You have to deal with that sort of situation proactively. Ring them up and explain their delays have pushed the project into a period where you have other commi

You have to deal with that sort of situation proactively. Ring them up and explain their delays have pushed the project into a period where you have other commitments. Ask how long it will take them to resolve their delays etc. and offer to reschedule the contracted time after that. If they screw you around later politely decline their work. If you don't respond to the blame they are giving you then they will continue to heap it on you. And that is not a working relationship.