These blocks aren't Duplo, and aren't MegaBloks but they look to be particularly suitable for younger children. The review on there mentions how very big they are which makes them a great introduction to building and construction games for young children. More so if you have older children who already have the smaller bricks in the house.
These John Lewis blocks* are reduced to £8.75, from £25. I think that's a Christmas gift sorted right there.
I also really love My kids would really love the Farmhouse and Tractor*. It's reduced to £17.50 from £35 and looks like it has loads of little characters and bits and pieces to play with. The reviews on it are fantastic too, five stars across the board. Despite the picture, this actually comes with one farmer and five animals but you can add extras from other sets.
Click and collect is free from John Lewis or Waitrose and standard delivery is £3.