thanks everyone for your advice.
Bozita do a tinned kitten food [chicken, shrimp, salmon and beef] which I've only found available from Zooplus. I started introducing it very gradually a couple of weeks ago mixing it with the whiskas but it is probably only in the last week that they have exclusively had Bozita. They were wormed a month ago and again this weekend [or at least one of them was couldn't get the other one to have the tablet]. I noticed the runs yesterday after worming but prior to that one of them had been a bit 'soft' but not runny. I had to take my moggie to the vet today for his boosters and took kittens along and vet said chicken or fish and to go back if things don't improve and to take them off the Bozita kitten for now [which we did last night]. They had some Bozita adult last night & this morning and chicken tonight. I don't think it can be anything plant or litter etc that has caused it as it is both of them. I know they've not had any human food as it is only myself and husband and neither of us has given them anything and neither have they had the opportunity to get at anything. I've an appointment for tomorrow at a different branch to see an eye specialist [my raggie Beau has an eye ulcer] and will take them then as well if need be but I don't think that I will need to. There hasn't been anything nasty in the tray today and both are smelling sweetly!