Thank you all for the advice. I suspected it might be a food intolerance issue (although I had no idea that Tonks were susceptible to delicate tums...)

I will take Sig to the vets shortly, and meanwhile will see what I can do about changing their food - I assume it is probably best to do this slowly so as not to cause more upsets?

Otherwise, I am pleased to say that my kittens are no worse off from being at the cattery for a few days - in fact, they were the most loving they have ever been last night!

I'll let you know what the vet says and what we have done to cure Sig!

Leigh