Paul, do remember when buying her the sleeping bag women tend to "sleep colder" than us blokes, so i'd be inclined to go for a 3 season bag, she can always vent it if too hot, but if she ends up cold it could spoil a lifetime of romantic nights under canvas*. 
*More likely silnylon but how unromantic does that sound. 
Include a little history in your walks. Pecsaetan - Ancient Derbyshire, Staffordshire and South Yorkshire - http://pecsaetan.weebly.com/