How can I fix my toilet ?

My toilet is some 40 years old with no manufacturers name on the ceramics, it does however have Shires Twin Flow Syphon printed on the flushing system. Traditionally this toilet flushes quietly but efficiently, but lately has failed to take away all the waste giving the impression that there is a partial block in the swan neck of the pan. This has been plunged and the sewer also checked and found to be clear but the problem still exists. I am now told that it must be the syphonic bomb at fault. The flush delivers the full amount of water in the cistern and on that basis I find it hard to believe that anything between the cistern and the pan is at fault.
Is it true that the problem is likely to be the Syphonic Bomb ? Is the part still available and if not can it be dispensed with completely and what would be the effect of doing so ?