Thanks guys, will try cleaning the air holes on the grass box - didn't realise that would make a difference. Also will check to see if bolt holding on the blade is tight.
Only problem with using the uncollected line as a guide is that u have to mow the entire lawn in one direction only, so u get to the top of the lawn and u have to walk back down with the mower idle.
I could rake it, its just the mower used to collect it and would prefer that it did rather than having to go around after it.
Will try tightening the bolt and cleaning grass box as next step.