Fiona - Gabby was swaddled until she was 10 months old (I know!! Sheesh!). Fortunately for me, because she was a March baby we didn't have the issue of warm weather for very long.
I have heard that the muslin wraps are meant to be cooler. Another thing that I did was use a single bed, flat sheet. I'd strip her down to her nappy and singlet then wrap her in the sheet. Seemed to work a treat! We also had an oscillating fan in her room on really hot nights just to keep the air moving around the room and keep her a bit cooler.
We're still in sleeping bags (she's now 18 months old) so I have the joy of trying to get rid of them before summer hits! It's a never-ending battle!! LOL