Thanks for your help, empowered mama.
Generally, DD wakes from her nap and whinges for a bit, then lays still for about ten minutes quite happily before shouting for me to go and get her. Even on a short nap, she isn't grumpy. If it was up to her, she'd quite happily have two short naps each day.
DD's sleepy cues seem quite inconsitent, to be honest. When she was younger, she'd yawn when she was tired. But now she'll do it when she is nowhere near tired. So I tend to clock watch, or wait until she is really grumpy. Perhaps she is over-tired when I put her down??
We seem to be completely out of routine now- always with a short nap in the morning and, if we're lucky, an hour in the afternoon. Maybes I should try 3.5hr for a few days and see how she gets on?? Really not sure, to be honest.