They keep moving the goalposts. As things stand, I have been told by the DWP that i am entitled to a full pension based on my late husband's contributions and need not buy any more years. THEN the new plans on the government website for flat rate pension said that i wouldn't be, THEN the website changed and said there would be interim arrangements and I would be entitled. The nice folk at the DWP have told me that they are not usually told anything until the new law is actually in force and the pension calculator software comes after that.
As things stand, the new rules and interim arrangements seem to depend on how old you are.