I just received an email from LBS Garden Warehouse, who we've always found to be excellent value and quality, saying that polycarbonate cold frames are reduced from £60 to £40.
http://www.lbsgardenwarehouse.co.uk/Cold-Frame-with-Polycarbonate-Glazing-PRDCF27/ (http://www.lbsgardenwarehouse.co.uk/Cold-Frame-with-Polycarbonate-Glazing-PRDCF27/)
They look to be a good size. Might be of interest?