I replaced the o-ring and leak has stopped.
Thanks to Bigmurph who suggested a part # and generously supplied a spare. What a guy - he even mailed it to me.
The o-ring appeared to have a thicker cross section than the original - but that may be because the old one was on the pump for 9 yrs.
Part # is M83461/1-348
If you google this you will see that
www.amazon.com
sells them.
Thanks to all for their suggestions and to my new buddy Bigmurph.
Al