Thank you all for your kind words.
Last night was horrible. I've never seen anything like it, her face was so swollen that her eyes were almost completely shut.
Her head was huge and really puffy. I left my camera at work unfortunately, I would;ve taken a photo as its hard to describe the state she was in.
We got some Zyrtek in the end, which after Googling seemed to be okay (I was willing to try anything her face was so bad).
She's not perfect this morning, her muzzle is still swollen and her eyes look sore (the only way I can describe her eyes would be like English Bull Terriers - those little triangle eyes). But she's much, much better than she was at 2am. She is okay in herself, if a little worse for wear.
As for the cause, I really don't know. I've not seen any bees or wasps about. We have had grass snakes in the garden before but not recently.
Thanks again for your kind words.