Quick update on this issue (for anyone that may be interested!).
I took Sig to the vets last night - he does have a slight temp and a loose tummy still - you will be pleased to know that the vet pretty much gave me the same advice as everyone on here!
He is on a starvation diet (unfortunately means that Mai, his sister, isn't getting much food either!) for 24 hours; has had a long lasting antibiotic injection and then is on small meals frequently of chicken and rice RC Sensitive pouches for a few days. I am then to introduce whatever food it is I want them to stay on slowly and see if his tummy copes with that.
I'm sure he will be fine (apart from reminding me of a slightly Asian "Puss in Boots" from the Shrek cartoon when begging to be fed!).
We go on holiday on Wed (to go and see some big cats in Africa

) so I am hoping he will have improved by then.
Thanks you to everyone once again.
Leigh