Reasoning and Problem Solving Questions Collection - KS1 and KS2:  TES Recommended
Resource White Rose Maths Hub: These booklets each contain over 40 reasoning and problem solving questions suitable for KS1, KS2 and KS3 classes. The answers are provided with some simple notes at the back of the booklet and for some problems supplementary questions and variation has been provided.
Puzzles for Year 6
PowerPoint Great selection of puzzles for Year 6 or UA Year 5.
Puzzles- Maths Zone
Website Great selection of interactive puzzles and problems solving from Maths Zone.
Website PUZZLEBOXX is a collection of puzzles and problems suitable for children (aged 9 & upwards) and adults.  Some are fairly simple.  Others may take some time.  Some require pencil and paper.  Others may require a practical solution.  Almost all of them will be easier to solve if you discuss your ideas with a partner.
Working systematically- nRich

Finding All Possibilities Upper Primary

These activities focus on finding all possible solutions so working in a systematic way will ensure none are left out.
Free problem solving- Maths Sphere
Website A great set of maths puzzles for upper primary children. All answers are given. Many teachers use these as a weekly challenge. They are ideal for printing out in colour and laminating, making a long lasting resource. These puzzles can also be found in MathSphere: It's All Figured Out!
Problem solving- Year 5 and 6 pack-TES
Resource An excellent pack full of puzzles and problems solving activities for Years 5 and 6.
Study Ladder
Website Lots of resources from this Australian site- but you must join.
Third Space Learning
Website  Mainly aimed at Years 5 and 6 , with useful resources, but also a website to keep an eye on as they will undoubtedly expand more .
Mathematics Mastery
Website Some examples ( full flip charts  and lessons ) of a Mastery approach.