I agree with what you say MacB in that we forget that the system we now describe as capitalism requires state intervention and rigid social policy to maintain it. The balance, as you say, is disrupted when the ideology of the laissez-faire stops paying the bills and hence its own responsibility to the system breaks down, leading to further differential between rich and poor, as is happening now. I rather think however that the social democracy you describe to 007 has already been tried and tested and found to fail also, given that redistribution only seems to go so far to bridging the gap between the richest and poorest sections of our own society and less still that of the international community.
Hmmm, this could get interesting, firstly the highlighted bit, I was talking in general and worldwide not specifically about the UK, as I think we've made more of a hash of it than some others. I also think it's all too easy to look at the negative state of play we currently have and extrapolate backwards to the idea that a capitalist/social democracy mix can't work. I feel this overlooks, or gives too little merit, to some of the enormous achievements this system has brought us. Universal healthcare, universal welfare and universal education are the three biggies that spring to mind.
You could spend all your life reading the theories about what went wrong, when and why - personally I favour a combination of vested interests and the media they control versus the total lack of cohesion on the left and a rigidity that meant they were unable, or unwilling, to be proactive in seeking alternates and working compromise.
The question of what would, or could, work in a socially democratic system only needs to look at the best of times and the factors underlying these. The part that shines out is 'Universality' which is why I favour something like a BI, 'rationing' of the housing stock and re-nationalisation of the essential externalities that benefit us all - energy, water, and transport. However I'd want to see more of a performance related pay aspect within these nationalisations, to keep the ability to pay and retain talent that works. If these basics are guaranteed to all then we can afford the laissez faire attitude to the rest and allow free marketeers a free reign in that respect.
I've deliberately ignored wealth/capital above(barring the domestic housing supply) as the hoarding/misuse of such is an international, not a national, issue and needs the appropriate level of co-operation. But it needs a lot more than the farcical latest agreements with Switzerland, wealth needs to have the tables turned on it. If it's all honest and above board then there should be nothing to hide.

But I'd go a step further and do away with the concept of a company as an equal, or better, of the individual. Do away with any and all forms of business taxation and place the entire tax burden on the individual with strict controls on capital movements across national boundaries. The latter being something that would/could benefit the third world far more than existing legislation. This should have the effect of shutting down loopholes, tax havens and the ridiculous practice of multiple shell companies. The word 'universality' arises strongly again here, without that these methodologies are bound to fail. As soon as there are 'competing' tax regimes then there is the desire to arrange wealth to benefit from these.
This all may seem a long way from what we have but the actual changes required would be pretty small and the impact would be positive for most of us. Those negatively impacted would be the ones that could afford it anyway, it wouldn't diminish their quality of life despite the amount they'd whine and moan.