How do I replace the belt on a Hoover Pure Power vacuum cleaner?

I have been given a Hoover pure power cleaner that is approx 4 years old and I think it needs a new belt as it doesn't pick up very well on normal use, but there is a strong suction on the tools. I can't find any screws to undo the bottom of the cleaner to replace the belt and I have no instruction booklet with it.

I notice the brushes are not turning underneath when I switch the hoover on, (yet on my old hoover when I switch it on, the brushes are whizzing round and it picks up well.) I am assuming the belt needs replacing if the brushes are not turning and it's not picking up. Or is there another reason it isn't picking up? The bag is nowhere near full and there doesn't appear to be a blockage anywhere.

It's a Hoover Pure Power 1500w model U3350 001 type VU01