I cant help with Wenduns links Im afraid, I dont think this is right or left wing thing, most people who object to mass immigration in my experience are left wing.
My central point is that no-ones knows how many people have entered illegally, but I think we all knows its considerably higher than what we would like to believe. This is down to the link I provided and the fact the people have genuine concerns regarding mass immigration.
Keep the figures lower to keep people happier