April 2002, which is exactly why I replaced my windows in February 2002 (and put central heating in). However, very few window installers were signed up until a long time after that date anyway. I spoke to mine probably some 6 months later, and he was just starting to think about it.
I did however make my systems all conform to Part L except for the K glass, which is a complete waste of money. IIRC, the payback for K glass was something like 50 years, and that didn't take into account the extra lighting needed because of the slightly darker glass. That money went towards a condensing boiler and extra insulation, which was a very much more effective use of it.