Primary Maths Challenge page.
There is one online PMC that is automatically marked at the end of the test. Try it out and let me know your score! Don't forget that this is not a mental maths test - you are expected to have pen and paper handy to do many of the calculations.
You can also download past paper, but make sure you try each paper before looking at the answers!
Please note that the above links are all generated from a database, so if the direct links seem to go to the wrong page, then go to the MA's homepage and click Primary.