I've since spoken to the young gentleman and he adamantly denies he was speeding. But it seems that in Spain it is only the Guardia Civil's view that matters. He also has no intention of ever going back to Spain again, and they haven't made him pay yet (this all happened nearly a year ago).
In a similar vein, when we were in Spain last week, my husband kept his headlights on at all times of the day and night when driving, so that he could not get caught like one person did on this Forum, being fined 200 euros for allegedly not having his headlights on during late afternoon. We also have to do this ridiculous stopping in the middle of the road wherever the authorities have chosen to put stop lines, when give way ones would be far more appropriate. It's a bit of a nasty game and you can be blasé about it until you get done for something unfairly. Some people can't empathise with things until they've faced the exact same scenario themselves.