I need a load of 100mm celotex insulation for my loft. Does anyone know where i can buy it cheap?

Building control say it has to be 100mm celotex to meet the U value

It depends on the quantities you need.

Have you tried Sheffield Insulations?

http://www.sheffins.co.uk/

Slight seconds / slight damaged pallets often come up on ebay, but delivery tends to be pricey.

Mike