I am astounded that Which? used the fact the dust bags could not be re-used as a negative? I know this was the era of open top bags which could be re-used, but by saying what they did they made it sound like bags SHOULD be re-used!
They most certainly should not. Once a bag has been filled once the pores in the paper are already clogged with micro-dust particleks and emptying the "physical" dirt will not restore 100% of the air flow, thus rendering it a very risky thing to do repeatedly if you wish your vacuum cleaner to perform well and (in the case of clean fans) last a long time.
I have to leave now for an interview, but will post more later.