Citizens Advice/legal advice seem to be good starting points deb.
The fact that the children’s father and his family drink heavily means that the children may not be safe all the time, that’s why I mentioned SSs or NSPCC before. It’s worth taking legal advice as to the advisability of this route.
You made a promise to your daughter to look out for the children, and you are clearly trying to do that. The children’s mother obviously knew the situation and wanted her own parents to be there to support her children.
Please try not to despair, there are some good and well meant suggestions on this thread. Please keep trying via these routes to establish contact with your grandchildren. I know that when children have had very regular contact and support from grandparents and they suddenly have it withdrawn, the law and SSs view is normally that the children benefit from continuing that contact.
I wish you good luck, and I send you love.