You may want to try bypassing the ignition switch and use a remote starter jumper with a pushbutton to bump the motor. This setup applies full battery voltage to the solenoid and should tell you if the internal contacts of the solenoid are good or needs cleaning.
Historically, this issue with the starter motor has afflicted many Europa owners. There was a thread about this several years ago on the Yahoo list. All four of the Europas that I've owned have had this problem. It was a crap shoot whether the engine would turn over or I would just hear the click of the solenoid when I turned the ignition switch on. Just this year, I installed a remote start relay to two of my Europas, a twin cam and a S2, and so far for the first time ever, both now start the first time every time.
I'm just wondering how many of you have or had this issue with intermittent starter motor problem.