do a search on the green building forum for triple glazing – they have dug out some good (cheaper and good quality) suppliers.
FWIW, I would look carefully at Triple Glazing – only worth it if a) rest of the building envelope is to super-duper insulation and air tightness standards and b) you are North of about the Peak (As a rough rule of thumb). Assuming these windows are going on a new super-duper low energy house, then they rock 8)
I did not know that a single (crap) brand and glass model was specified in the UK…. Pilkingtons K-glass also has less heat keeping properties than many of the competitors, lower light transmittance and a right odd smokey colour.