Myson?

A family friend who is a widow wih a C/H boiler problem. Too far away for me to check at this moment in time can anyone help with what the problem is.

When the boiler comes on the boiler header tank overflow starts to flow water after about 10mins then continuous a steady run of water. The ball valve is ok stops at 35mm below overflow, obviously only when boiler is off.

Myson floor standing Model I think is Marathon 80 approx age circa 1996 at a guess.

Many thanks in anticipation.