I guess he's saying its good to question and scrutinised any report commissioned by any body that has an interest in getting a specific answer.
Even when someone does have the balls to offer an independent opinion and not what the Government wanted to hear when they commissioned it, they just get rid of the guy and bin the report (remember the report that said cannabis should be legalised and they sacked the guy who wrote it?)
Don't forget these reports are not the same as the scientific process where what you say is peer reviewed and you have to gain consensus before it is regarded as true.
Actually, a majority of these reports in general are scientific research and are published in peer-reviewed academic journals. Now government bodies do request specific reports and will typically request these from Academic professors who have published in the relevant fields. A summary report may then be provided to the government that isn't itself peer reviewed , but most of the data and findings within the report will almost certainly be taken form prior peer-reviewed and published research, with citations provided.
In the specific case on immigration impacts on mean salaries, there is a scientific consensus of relatively insignificant effects and potentially positive effects, especially form EU-migration which is actually composed of a lot of skilled workers, in fact there is a mix on skilled and unskilled but neither group makes a negative pressure on salaries.
There are a few key points to remember:
1) The minimum wage is a hard lower bound on salaries. the completely unskilled menial labors are paid minimum wage, it makes absolutely no difference if this is a British citizen or other EU citizens doing the job at min wage, there is no change in mean salaries.
2) Skilled workers get paid the market rate, and EU migrants on average are more skilled than British workers - more likely to have a degree for example. people tend to think of factory workers and farm laborers when talking about immigrants form Poland but they are also going to be software engineers working for investment banks in the city, etc. Employers of skilled laborer pay market rates, and the companies are at competition each other to retain the best employees, which is what drivers up the salaries in the first place. There is absolutely no advantage in paying a good Polish web designer less than the rest of the employees, he will just move on to a different company that gives a fair pay.
3) Many companies have fixed pay grades, age,, years experience and education are simply plugged into a formula with the job type and a salary figure produced. This is especial true for public entities like the NHS, which is one of the largest employers of immigrants.
4) it is illegal to discriminate based on race, nationality, place of birth or religion. Paying someone less because they come form Romania could land the employer into legal issues.
5) Most people simply aren't racists and discriminatory so wont differentiate salaries based on nationality.
6) Immigrants just like anyone else want to earn the best wage possible, why work at £12 an hour for company X if company Y offers £15? Immigrants tend to support each other in getting good results. If an immigrant software engineer is earning £55K at one firm in London and his immigrant friend is earning £40K at a different firm, if a suitable position is opening you can be sure the former tells the latter and tries to get his mate an interview at the better paying software firm.
7) The whole of the EU is an open market, immigrants don't only go to the UK, they also go to countries that have a higher minimum wage than the UK, or different markets with different salaries. If the average UK salary for a particular employment area is lower than France or Germany then many migrants simply wont go to the UK, and market economics will mean the UK salaries may actually increase.
8)Immigration provably increases GDP and GDP per capita, which ultimately increases average net worth and average salaries.
Finally, the solution to prevent possible wage depreciation due to immigration is absolutely not to reduce immigration, which would have a large negative effect on the economy. Instead the UK and or EU could bring in legislation to make it illegal to pay a foreign national less than the median salary for that job in the local area, and have the employers have to prove to a governing body that the salaries are all aligned. The US does this for example. however, it is mostly a waste of time because as proven countless times, immigrants dont negatively impact average salaries so it is extra red tape just to appease the xenophobes.
