cistern internal overflow too short

[Addis]

The flush rate is at maximum already, the cistern almost empties when I flush.

As I mentioned previously, if I block the overflow and add more water I get a good flush. As the cistern only holds 6 litres when full, being 2 litres short is giving an inadequate flush.

I'd rather not replace the syphon as, being a consealed cistern, access is difficult.

 

[Charnwood]

It's not a syphon, it's a flush valve.

I think you have 2 options; continue trying to botch it or replace the flush valve.