In 2006 Royal Mail conducted a complete overhaul of the pricing structure for inland letters and on 21 August introduced pricing based on size as well as weight.
"Pricing in Proportion" split 'letters' into three types: letters, large letters, and packets. Tthe price of sending a standard item did not change, but letters that exceed 240mm length, 165 mm width, 5mm thickness and 100g in weight were charged as Large Letters. The upper limit for Large Letters is 353mm long, 250mm wide, 25mm thick and 100g weight.
Items above these size limits are treated as packets. This new pricing structure better reflects the actual cost of handling these items as they are too large to be sorted by machines. The weight limit for standard letters will rise from 60g to 100g.
New Large Letter 1st and 2nd class definitive stamps were introduced from 1 August 2006 and 1st and 2nd Class definitives for standard letters will be redesigned with a new larger service indicator. The issue of booklets, coils, etc progressed through 2006, but the Large Letter business sheets were not issued until 2007. Issued 27/03/2007 - Large Letter business sheets 1st & 2nd
SG 2657 (same number for booklet and business sheet stamps)
Printing date 22/01/07