I brought a Matsui TV the other day and now I can't seem to gat any radio channels or anything on the red button. The main terrestrial channels are ok but some of the smaller channels are just showing a broken image with jumpy sound.

Annonyingly I had a freeview box which worked fine up until a week ago.

Is my telly knackered? or is it something else? And is it easily fixable?

Sounds like a poor signal rather than the tv at fault.
Try it on a different aerial.

yep sounds like the aerial, worth a try retuning it though. Turn it off at the mains for a minute or so then retune it as you did when you first set it up.