Part needed for Roca Close coupled toilet cistern?

I fitted a new inlet valve to my bottom-filling Roca toilet which is about 12 years old. It is about 2cm shorter than the old one.

The problem now is that the water level is not enough for the toilet to flush first time and I have to keep flushing several times before it works. I replaced the diaphragm in the syphon but I think the vaccuum in the syphon is too great for the water to be pushed rould.I can't find a new syphon small enough to fit in the cistern. I've tried numerous sites to find one without success.