Where I am a Governor, I have observed Maths lessons in Y5. Children sit on tables of 6 and work with other chidren, work in pairs, or work by themselves. The lessons are taught by the teacher to all, usually via the whiteboard, then children practice what they have learnt and this is initially with all the class joining in during the lesson. The work is then differentiated with the bright children doing the hardest or even Herculean tasks. There is also Hard and Harder so a bit like your school. However, they choose, but with guidance. The teacher circulates around the room to ensure the children are doing what they should be and challenging if necessary. The vast majority of children choose appropriate work and I have noticed some go straight to the hardest and then have to go back a stage! Children who regularly achieve at the highest levels are set addtional work within the topic to extend their knowledge. They do not start another topic.
All the work is in their books and I frequently review books with the teachers to look at progression. The National Curriculum for Y4 is not the same as Y5. If there is no work in the books, then complain - this is not right. What would an Ofsted inspector say? It sounds too fluffy to me.
The pace of the lessons is fairly fast but the children who may get left behind do pre-topic work to brush up on tables or whatever they need to access the new topic. They are taken out of another lesson to do this. However, the new curriculum stays on a topic for longer than the old one, so may appear slow in comparison.
Very few teachers are trained at the moment to teach Mastery Maths. The project is being rolled out and the first round of teachers are now doing the course, I believe. (At least in my LA). Unless your Maths co-ordinator has been chosen, then you may not technically have Mastery Maths in your school. It is a steep learning curve and not all chldren will be given challenging sophisticated questions. Some will need to practice the less sophisticated stuff first, then progress to more challenging work.