This does sound like it's part of Agenda for Change, you don't work in Radiography or Radiotherapy do you?
The simple answer is yes, they can do this. The hours are paid, they are part of the full time working included in Agenda for Change. These hours have been phased in over time in order for people to get used to them.
Agenda for Change was done in agreement with the unions and the professional bodies that represent and as such then yes it's legal. It's a replacement of the Whitley system so anyone previously on a Whitely contract is automatically switched to Agenda for Change, the only way you might get around it is if you were on a Trust contract but even that I think would be unlikely.