Can someone help me with some real data on the 'building more roads doesn't solve transport problems' view? I always see it as common sense that empty roads will be filled and that an entire change in attitudes to travel, for work or pleasure, and provision of alternatives to the car, is required to really sort things.

Not sure what kind of real data you're after, but road building has been the main focus of transport plans up until very recently. In 1989 one particular transport plan involved over 500 new schemes for roads, in 1998 a similar plan involved 37 schemes.
So this means that we aren't as besotted by road-building as we once were. As you said, all that happens when you build new roads is that more traffic comes and fills them - and we're now witnessing the effect of all that huge road growth between the 60s and the 90s. The number of cars on roads in the UK has increased by 73% since the late 80s, i,e since those 500+ roads were built.

I don't think many people will give up cars out of good will and morality, but they might give them up when driving becomes so stressful and roads so jammed that any alternative seems better. So by not building more roads, we eventually squeeze cars out of the picture.