it really depends on the room your lo is in. ds's room is freezing in the winter and boiling in the summer. when i enquired in a local baby shop for a 0.5 tog grobag,i was told "oh,you wont need one of those in this country it doesn't get warm enough" i got one from bump-2-3 and used it throughout the warm summer nights (the year before last),when ds's room regularly was 23-25 degrees c even at night with the windows open.
needless to say,summer grobags dont get quite as much use as a 2.5 tog one,and ours were in fab condition when i sold them on here. check out the grobag section in the for sale, you may find summer ones in great condition for a fraction of the price of new ones.