Well, I don't know about the Avon Road Riders, but I will say that 18psi is way too low for Avon Speedmasters.
I had my front Speedmaster go down to 18psi one time after letting the bike sit a long time,and the damn thing was so squirrely I had to find an air pump at a gas station as fast as I could.
28psi in the front, with Speedmasters, is a good pressure.
I don't know who decided that 18psi was a good figure to put in the manual, but I suspect it's a typo, if it actually appears in the manual that way.
That's ludicrous!
I find it hard to swallow that ANY road tire would be specified to operate at 18psi.
If people are running that pressure, I would suggest much higher pressures be tried.
That might very well be the handling issue right there.