I agree that Bowie / Eno was a huge influence on the genre though if you listen to the track 'My Sex' off the first Ultravox album you have to concede that the Numanesque 'automotan' sound was born. Having said that, the first two LP's are schizophrenic in that they show a band with one foot in punk and the other edging towards the euro-synth scene. As you rightly say, by 'Systems of Romance' they had found their sound.
I remember John Foxx taking a lot of stick for his first solo LP (Metamatic - with the singles 'Burning Car' and 'Underpass') for copying Numan. Very unfair. Foxx had been pre-occupied by JG Ballards works for some time ...a rich seam of imagery borrowed by Numan and who then made it his own.