Can't fit bag into ProAction 1400 VC2940RD?

Hi,
We have been given a second hand Pro Action 1400 vacuum cleaner (VC2940RD). When we got it, we tried it out and the suction was great although after just a few minutes we realised that the old bag which was in it when it was given to us was full and disintegrating.
So I sent off for a pack of identical bags. I have just tried to fit a bag and just can't work out how it goes. It seems almost as though there is something missing, there doesn't appear to be any way of holding the bag in place.
I have searched online for a user guide/instruction manual, but can't seem to find one. This may be because the brand appears to be one sold by Argos/Asda and it might in fact be manufactured by another company, though I have no idea who and can't seem to find out either.
Any suggestions either of where I could find a manual to download (any file format would be alright) or how I can solve the problem would be most gratefully received.
Many thanks for reading this,
Best Wishes
x