Cistern overflow

Defensivelynchpin

It's an old Skylo bottom entry fill valve,  diaphragm was is probably perished or split. ( made by Viva)

silvercar

Defensivelynchpin said:

It's an old Skylo bottom entry fill valve,  diaphragm was is probably perished or split. ( made by Viva)

At the bottom of the blue thing on the left or the central thing?

Defensivelynchpin

Under the blue part at the top of fill valve

ThisIsWeird

Don't try and dismantle until you find the instructions.
'Easy' when you know how, and a lottery if you don't.
Def has given you the make. 

silvercar

To conclude, plumber came out and changed the part. Of course, the toilet started behaving when the plumber appeared, but I got it changed anyway. He then had a slight problem putting the lid of the cistern and button on the top back without causing some water to flow into the pan. I do wonder whether the cistern had got knocked and the middle piece wasn't sitting completely level and this was what the problem actually was.

Ganga

I dont think so because normally the water level is well below he overflow level.