Changed toilet behaviour

Ok so Tiddles is nearly 4 months. About 2 weeks ago I changed his food, he loved it, when I changed it back to his regular food he had diarrhoea. He then had his second vaccinations with the vet so I spoke to her about it and she said it was the change in diet and to give him plain Cooke chicken to settle it. He had that for a few day so I put him back on his regular cat food (felix) then the diarrhoea started again. However, he's not making a run to the litter box, he's using towels or anything that is on the floor. He seems to not go unless I go to the litter box with him? He is always put downstairs at night (otherwise I get woken up at the most ridiculous times) he does use the litter tray during the night but why the no attempt to get to it when I'm in the house?

Perhaps he is trying hard to tell you something.  How else can he tell you that the new bag of Felix cat food is upsetting his stomach?  He is showing you!  Listen to him.  So either the new bag of Felix you got is bad (and this happens, where do you think all the recalls come from?  Each new bag is a new run, and sometimes they get contaminated, and make a bunch of animals sick, so they get reported and eventually recalled.) Or else his stomach liked the other food better and does not like the return to the old Felix cat food.  Anyhow, since the food you had him on before agreed with him, and the bag of Felix you have obviously does NOT agree with him, why not just put him back on that right now and see if this diarrhea clears up?  And hold onto the bag of Felix, search the internet for "complaints about Felix cat food" to see if any other people start having problems.

I agree--Felix is showing you what kind of food her prefers.
I'd keep the old bag, just in case.
I'd also read the ingredients with a magnifying glass (and, sometimes that's the only way you CAN read the ingredients) and see what the major differences are between the two foods. Though sometimes the tinest, least used/amount ingredient can be the culprit, like carrageenan.

I just searched for complaints about Felix, and noted it is also a Nestle-Purina product, and yes, there are suddenly a lot of complaints about Purina foods making cats sick, on the U.S. Consumer Affairs site (http://www.consumeraffairs.com/pets/ralston.html) so if I were you, I would go get another brand of food, especially since his problem cleared up when you had him OFF that food and on chicken at your vet's suggestion, and now that he is back on the Felix food, the problem is back.  That pretty much points at the food being the problem.

I was always told to steer clear of Whiskas cat foods, because of their additives, but that was the change in the middle. However I also took another suggestion and kept him off food fr 12 hours to see if it was a tummy problem and would clear up. Then went and bought a much more expensive food for him with no additives, wheat, etc, proper stuff. This morning I wake up to a big puddle of it on the kitchen floor.

Be really careful with the no grain high protein food. I know it's said to be better for cats but I put my cat on one and he did ok at first but then developed diarrhea with blood streaks. I didn't know what the problem  was and someone on another site told me to get my cat off the food NOW because hers nearly had renal failure (2 of them) after 5 days of being on the high protein no grain food. She said she'd read all the same things I had about it being better. But some cats systems are just really sensitive and don't do well on it. Each cat is different. I have read a few stories of people whose cats had diarrhea on the stuff and when they put their cat back on a regular "cheap" cat food they got better. So please be careful. Sometimes it's just too rich for a sensitive system. Maybe you could try just feeding him one brand of canned wet food for a few days, no dry food at all, till his system calms down, then gradually give him a little of something that is just a regular cat food, no dyes, or too many additives, but definitely not the grain free. The grain free has pea protein, potatoes etc that can irritate some cats more than than the grains and corn do. I bought Authority brand cat food for mine and he is now doing well on that and Friskies canned cat food. The diarrhea went away when I took him off the grain free stuff.