this is from the bumpto3 website:
Why is the tog rating of the cot bed size duvet different to the tog rating of the single bed size duvet?
Because the size of the cot bed is smaller it automatically retains more heat when the child is sleeping, however, the larger surface area in a single bed means the child only heats the area he/she is sleeping in, therefore, a higher tog rating is required on the single bed size. The single bed size comprises of 2 x 4 tog duvets that popper together giving flexibility to reduce the tog to 4 togs when the nighttime temperatures
Why only 2.5 togs when my winter weight duvet is 12 togs?
A. Health professional guidelines recommend that baby sleeping bags should be ?lightweight'. Our own research and our mounting body of "parent testimonials" confirm that our 2.5 Tog grobag® baby sleeping bags are ideal for the UK climate. A baby sleeping bag surrounds the baby, providing insulation from both below and above, and also a warm pocket of air around the baby.
Because of the low tog rating grobag® baby sleeping bags also provide a versatility that a higher tog sleeping bag would not allow, and both our 2.5 tog and 1.0 tog baby sleeping bags can be used for the vast majority of the year.
When the weather is very cold, simply add more clothing (this could include a sleepsuit, socks and even a cardigan). When the weather is warmer, or perhaps if your child is unwell, you can remove layers of clothing while still keeping your baby at the right temperature. Never use a duvet or quilt with a baby sleeping bag.