If it's within the year, send it in for repair, complain that it emits a high pitched whistle.
Thing is, NTL STBs are better off not enclosed, that way you can pull the power lead easier when the front panel reset fails after you have mistakenly hit the self destruct (red) button. And yes, the TVs are rarely designed to allow easy placement of set top objects, I think this may have something to do with people placing vases etc on top of the TV and watering the inside of the TV when they water the contents of the vase. (electrical appliances are run on smoke, when you see it escape from the appliance, the chances are it won't work so well)
As a matter of thought though, what make and model is your old TV? (model is normally on a sticker on the back) I may be able to point you in the right direction to repair it
[edit] looking at the picture, it looks like a Ferguson TX series, or the ITT version of it... usually there are a couple of variable resistors on the board inside it, one marked VERT and one marked HORIZ which govern picture size, if the vert one fails the picture drops to about 1 pixel high, and they are easy to replace