Usually a very simple repair, although it will require a TV engineer.
One (or possibly two) parts called positors, probably need replacing.
The engineer will likely take about 15-30 minutes. Expect a bill maybe £50-£70.
Essentially, the shadowmask of the cathode ray tube has an uneven magnetic charge.
Model Number is on the rear of the set
in the format TX-#####
If you provide the model number the engineer will be able to bring or order the correct part and save you time.
(NB. Probably not the cause of your problem *but* always keep any magnets or items containing magnets (eg. loudspeakers) away from the immediate area of the TV.
Cobweb
January 2005