Hi touie, thanks for reporting back!
Hm, good to know it wasn't just me. Still irritating.
You can get decent elastic cord in a climbing shop - I got some for 60p/metre in Lakes Runner/Lakes Climber in Ambleside - you'll want a couple of metres (more than you'd think).
I thought mine didn't fit wonderfully either, to look at, but I ran wearing it for more than two hours of Sunday with no ill effects. And I was carrying probably 2.5 kg (pack, water, food, emergency gear).
Do with the manufacturers would build these things with chest straps fitting a bit higher to suit the female anatomy! That's the reason I didn't go with any of the Inov8 packs.