It does indeed.
To start with, the Wikipedia page on radon is pretty great (it's rated a "good article").
Now for facts: Radon was discovered to be a problem (woohoo, 1960's Time article) through studies of miners back in the 1950's, when just a few too many non-smoker miners started getting lung cancer. These were all uranium miners, and as radon is a direct byproduct of uranium decay, one thing led to another (emphasis mine):
To estimate the risk of lung cancer mortality among nonsmokers exposed to varying levels of radon daughters, 516 white men who never smoked cigarettes, pipes, or cigars were selected from the US Public Health Service cohort of Colorado Plateau uranium miners and followed up from 1950 through 1984. Age-specific mortality rates for nonsmokers from a study of US veterans were used for comparison. Fourteen deaths from lung cancer were observed among the nonsmoking miners, while 1.1 deaths were expected, yielding a standardized mortality ratio of 12.7 with 95% confidence limits of 8.0 and 20.1. These results confirm that exposure to radon daughters in the absence of cigarette smoking is a potent carcinogen that should be strictly controlled.
Adam Davis' commented link to the National Cancer Institute is certainly informative, as are a few pages created by the Environmental Protection Agency (EPA) (both links share a certain amount of information). The first (both, in fact) EPA link has a nice "risks chart" for smokers and non-smokers, showing the levels of radon exposure and the correlation to cancer occurrence. This chart will be better understood, though, if one knows something about measurement meanings (units are in this case, as in most cases when dealing with radiation, diverse and un-unified at best). Taken from the Wiki article above:
- Radon is often measured in picocuries per liter (pCi/L).
- The more "standard" (read: SI) measurement is becquerels per cubic meter, where 1 pCi/L = 37 Bq/m3
- The mining industry uses something entirely different, of course, called a working level (WL). Since this is where all the data initially came from, what we now use has been translated from it. But for consistency's sake, one month's worth of exposure to a WL, a working level month (WLM), is "roughly equivalent to living one year in an atmosphere with a radon concentration of 230 Bq/m3."
It'll be even better understood once one knows that, from the second EPA article, "The average radon concentration in the indoor air of America’s homes is about 1.3 pCi/L" (~48 Bq/m3). But some are going to be more, and it should absolutely be tested on a case by case basis (a figure the Wiki article claims in the Concentration Scale section is a universal rating of 100 Bq/m3 (2.7pCi/L) for a house, which is certainly higher than the EPA's recommended amount).
To wrap this up with some more modern statistics, take a look at the EPA fact sheet from a pooled study from 2005 (emphasis mine, extra parenthesis and spacing issues theirs):
Overall, the odds ratios for lung cancer increased with increasing radon exposure categories, with an odds ratio of 1.37 (0.98-1.92)) for concentrations exceeding 200 Bq/m3 [5.4 pCi/L] relative to concentrations under 25 Bq/m3 [.68 pCi/L].The overall estimate of the excess odds ratio for lung cancer per 100 Bq/m3 [2.7 pCi/L] was 11% (0%-28%).
That info appears to meld smokers and non-smokers, but that's fine. The standard claim (from almost all of the citations here) seems to be that about 21,000 Americans die every year from radon exposure, which is also a combined number (about 2,900 non-smokers, according to the second EPA page). One can quickly see that this 21,000 number is far less than 11% (which is good, especially since that's only the increased odds of getting cancer, not dying from it) - it's actually 0.007% of the overall population, or about 0.001% for non-smokers (given an American population of about 300 million). This shows that our houses, on average, are pretty good about having low radon levels. If we take the 1.3 pCi/L (48 Bq/m3) EPA estimate to be true, then we're at about half of the "universal number."
So even if you're a smoker, radon isn't going to enormously change your risk of dying from cancer (unless you live in a particularly radioactive area), but there is a definite and defined increase, and you'll certainly be more likely to develop the disease. It's a very easy "better safe than sorry" call to make.
(I apologize that all of this data primarily concerns the US and not the UK, but I'm sure this all holds roughly true anywhere in the world.)