From national stats for school age children:
There were 669,531 live births in England and Wales in 2006 compared with 645,835 in 2005, an increase of 3.7 per cent. The number of live births has been increasing since 2001 and the number of live births in 2006 reached the highest level since 1993 (673,467).
So it would appear that there were abut 15000 more children born in 2006 than 2005. Does that mean that immigrants have had 335000 children?? Am I being thick? 
Also, our local council published the following in their 'strategic plan' 2004-2009, even though it seems national birth rates were rising. Goodness knows how they got these figures, as another doc put out by the same council at the same time offered no projected figures, with the admission that population figures were difficult to predict:
Over the period 2000-2011 children
of primary school age (4 to 10) resident in the County are projected to decline by
6,500, whilst children and young adults of secondary school age (11 to 18)
resident in the County are projected to decline by 800, from a peak in 2005.