Loose blade on mitre saw?

Look at the blade collar behind the nut it will have two flat edges for a secondary spanner which should have come with the saw as well as the nut spanner.
Alternative type
Take a look on the motor near the blade end see if there is a shaft lock button press this in and the shaft can't turn while you tighten or release the blade securing nut.

Another alternative only a guess though.
Same as the angle grinder disc collars.

If the blade collar has two holes opposite side on the collar then a spanner with pins on is the one again should be with the tool.
Performance tools I believe were made for B&Q if probs go to their help desk and ask if they know or a telephone number for Performance tools.

Bob
February 2010