Hi,My name is***** will try my best to help you with your question.
The fault is likely with the power supply board, which will cost around £100 to replace plus labour.
Eventually it will completely fail.
I wouldn't spend that much on an 8 year old TV, since for a little bit more you could buy an equivalent new model.
If repaired, there is a risk that other boards could fail soon after, given the age of the TV.
I have found a refurbished power supply board available for £42: http://www.ebay.co.uk/itm/1-876-290-13-G5-A-1558-703-A-SONY-KDL-52W4500-POWER-SUPPLY-/331913798561
If you have some technical skills, you could swap in the replacement board yourself and save on the labour costs too.
Hope that helps!