This column prizes optimism: a sense that after less than seven decades of mass migration, the British experience is broadly a pleasing one. Look at it this way: we enjoy the absurdities served up by Ukip – whose talisman always seems to be the bigoted time-forgotten Major Gowen in Fawlty Towers – but we aren't, say, Italy or France. Last week a poll suggested that one in four French voters are ready to vote for Marine Le Pen.

What we know is cause for optimism: the problem exists in that hinterland between what the former US defence secretary Donald Rumsfeld called the "known knowns" and the "known unknowns". Two things reached my desk this week. One was research by the Centre on Dynamics of Ethnicity at the University of Manchester. Academics there looked at how the rise in private renting has disproportionately affected some ethnic groups. Private renting, it found, was highest in 2011 for those one might describe as "other white", ie eastern Europeans, and those census-classified as "Arab". It was lowest for the white British and black Caribbeans. Both are established enough to either have their own homes or, particularly in the case of black Caribbeans, to benefit from social housing.

But that brings us to the second development, which might help explain why so few black Caribbeans were able to avail themselves of private rented accommodation that year. It's the result of an investigation by the BBC. Suspecting a deal of subterranean racism in the rented sector, the Inside Out London programme posed as a private landlord and approached a string of estate agents. We want tenants, they said; no African-Caribbeans. No problem, seems to be the response from 10 agents. How does that grubby practice work? "We cannot be shown discriminating against a community. But obviously we've got our ways around that. Ninety-nine per cent of my landlords don't want Afro Caribbeans, or any troublesome people," said one. And they meant it. A white researcher asked to view the property. No problem. A black researcher asked. "Sorry sir, it's gone."

Optimism's great: it's probably necessary – but just as well to keep in mind the known unknowns. They occasionally come up for air and it's not a pretty sight.