EVF/LCD fault

[Daveysnap]

I would be grateful for your thoughts out there .A77 Mk 2 low shutter count. After doing some Macro work in which I took ISO up to 20,000 plus Inoticed a very thin bright yellow line dissecting the screen in EVF and on LCD. This changed to blue/magenta and prevalent in relatively low light. 
The line does NOT appear on the 'printed"photograph. It appears to be directly linked to the amount of light coming into the camera. It is present with the lens off as well and a change of battery has made no difference.
Very grateful for any thoughts and idea of cost ? 
D