Thanks both. I have always asked people to let her come to them with their hand out to sniff, which had previously worked well. These last two times she has done that and then growled aggressively whilst sniffing their hand, so I'm now worried about letting her say hello to any strangers at all in case she bites
I'm pretty sure she's going through a fear period, as she is showing other signs like not wanting to go down the alleyway and jumping when a jogger goes past. Her hackles have always been up when saying hello to strangers but she has still approached in her own time and enjoyed a stroke.
It's just working out how to manage it without avoiding interaction and therefore risking desocialising her, whilst making sure she doesn't bite. It's a bit easier when someone comes to the house as I can ask them to totally ignore her until she makes it really obvious she's comfortable, but there isn't time for that to happen when out and about on walks, which is where she meets the most people.
EDIT I should add that there is a possibility that she's about to come into season again (1st cycle was at Christmas) so I understand that can contribute to mood changes.