I'm going to suggest the opposite to other PP. I've used grobags for DD since she was 5 months old (she's now 2.5) and I'm using them for DS. I've found a 0.5 for really hot nights and a 2.5 for cooler nights. If you look on the temperate chart on the back (here's a link to the chart http://www.gro-group.co.uk/sizes-togs-and-what-to-wear/)
you can layer your bub's clothing to add extra warmth or remove layers.
*26 degrees - the chart says to use a short sleeved bodysuit with a 0.5 tog bag
*24 degrees - the chart says to use a short sleeved bodysuit with a 1.0 tog bag (you can also use a long sleeved bodysuit with a 0.5 tog bag)
*22 degrees - the chart says to use a long sleeve bodysuit and a 1.0 tog bag (you can use a short sleeved bodysuit and a pyjama top and a 0.5 tog bag).
*20 degrees - on the chart you can either use a 2.5 tog bag with a long sleeved bodysuit or short sleeved bodysuit with a pyjama top.
Its sort of hard to explain but basically I've found a 0.5 tog and 2.5 tog caters to all temperatures. On super cold Melbourne nights I've even put my DD in a Bonds growthsuit with flannel PJ's over it and then a 2.5 bag (no blankets).
I hope this makes sense, PM me if it doesn't.