Testing SaaS, all parties included?

This week I was at a meeting in The Hague where a SaaS-solution in e-government (GovUnited) was discussed by people from science, companies and the Dutch government.
The idea is to develop and maintain a standardized website for Dutch cities, which can be customized per city for its particular needs. The maintenance will be done from a central place in the Netherlands and the cooperating cities (the customers) will pay the service-provider GovUnited a yearly fee for development and maintenance of their website.
Next to this, GovUnited can act as a intermediate between the cities and other parties, like e-payment services (eg.Ogone) or other government services and facilitate the connection between both e-services.

This makes it for me as a testprofessional interesting, because with all these different parties involved, who is solely responsible for the quality of the SaaS-product?
If the website is running, but one of the links to another party (like Ogone) is malfunctioning, who is responisble for this, Ogone or GovUnited? Or perhaps even the party hired to develop the website?
Each party can develop and test their component of the SaaS-product, but who is responsible for testing the SaaS-product as a whole. This multisystem integration test must be considered in the development + maintenance and can't be just be planned and executed at the end of development because if things get wrong then (and most times it will) it gets nasty and dirty for all parties involved.

So, a careful planning of development and test should be made between all stakeholders to ensure the deadline can be made with possible risks taken care for.