Usually official published statistics. the figures for household size come from 'Social Trends' 39 published by the Office for National Statistics.
Numbers of houses on two specified council estates, personal experience, like counting houses with externally insulated walls.
Immigrants in high status jobs, well, go into any hospital, and I have been in many over the last 15 years as far afield as Hull and Southampton and it is impossible not to notice how many of the medical staff are from European countries, I have the same experience with GPs. I live near Oxford and within the Science Vale centred on Harwell and I know, again from personal experience how many of the scientists come from EU and other countries.
There have been many government reports and Select Committee reports (like that on Sports Direct) which are well reported across a range of media giving the information on the use of agency workers. Other reports from a a range of authoritative sources, including trades union and think tanks have reported on the problems of zero hours contracts..