Well blow me...
Took a glance at the screen a couple of hours ago to find just two tiny marks left. Checked again just now... All gone!
No shaking of netbook or three year old either
(Loved that comment thanks, spekesoftly!)
Cheers all. I realise it is liquid crystal but even so it is nice to have had something fix itself as well as at no cost (I've had a run of product failures in the past week)!