I just purchased a 28" Widescreen its a Toshiba not that expensive (350.00)

My problem is, when there is black or dark images on the screen i get a wavey effect.

Is there something i could do to improve this.

Is this problem only when using the scart socket inputs (like Sky or DVD) or on normal channels like BBC1 via your aerial?

If it's only on scart inputs (av1, av2) it might be that you need a better quality scart lead. One that has individual screening.. leads like this cost from about £12.

Do you mean a wavy effect at the edges of the picture in particular?