Now there's a surprise:
http://news.bbc.co.uk/1/hi/uk_politics/7415261.stm
Apparently the Office for National Statistics had estimated 43,000 short-term migrants in England and Wales between June 2004 and June 2005 but the committee claimed that was implausible since there were well over 200,000 new Nat. Ins. no. registrations in London alone!
The committee said "statistics for 2005 on international migration were based on
"a very small sample" of interviews with 2,965 people entering the UK and 781 who left.
That'd be about right!