Just got back from vet. Confirmed she's much improved. Temperature down to normal and not arching her back anymore. Special diet and antibiotics for the next few days. So what it was I guess we'll never know. I must say I do keep an eye on her when we are out and I'm not aware of her having eaten anything. Anyway the SBD is lying quite calmly at my feet at the moment. On the upside she's lost nearly 0.5kg.
What a relief, so pleased she's on the mend and not noticeably uncomfortable any more. Poor Lady, and poor David! Isn't it odd how these things sometimes don't have an obvious cause? But hopefully that doesn't matter and she'll be back to her nomal self in a day or two and your stress bucket can empty a bit