Thanks ladies.

I actually took her to the GP today but he said we just 'have to keep an eye on her to work out what it is'.

I might test her in a few weeks when her hives have gone and she's had a good few days. I will be close by to the hospital and armed with antihistamines. She's been well other than a little swelling and the massive rash of hives. Hopefully that's all it is and we can avoid them from now on (but that SUCKS because hubby and I LOVE mushrooms!).