It is 1100/1300 ?
It is valencia engine, it is a very known problem for cars doing short trips and using crappy/cheap oil.
They eventually break the rocker shaft, due to lack of lubrication.
Try some wynns flush, then refill with decent oil and get it nice and hot.
I used VALVOLINE in my escys and the inside of the r/cover was spotless, after this.
tabetha