I had suffered from similar symptoms since my first memories pretty much untill I was like 20 or so: tummy cramps, days of loose stool followed by days of not being able to poop followed by days of loose stool accompanied by gas so smelly it made me sick and so on. At that time I started a strict diet to lower my uric acid levels to prevent gout later on in my life. I had to change my diet completely and to my surprise my tummy started working properly. Bowel movement at least twice a day, no more bad smell. I didn't think my tummy was capable of that, since I basically grew up with it.
Since then I got stuck in this horrible IBS symptom loop only a few of times for a couple of weeks in last 10 years. I am in one right now, since a couple of weeks, so I am going to do my routine which helped everytime and certainly hope it will this time too.
Basically, what I found was that bad smell is related to how quickly the food passes through my body. This is related to what I eat and to how clean my colon is. The colon can clean itself, but for me it takesmany many months of high fiber diet -- similar to a diet I undertook when I was 20 to lower my uric acid. Meanwhile, however, I discovered that it makes my acid reflux worse (man I am a mess...).
Sorry, the previous paragraphs contain very little useful information, but I am feeling pretty sorry for myself at the moment. Let's skip to my routine.
1.) Psyllium husk powder: 1-2 table spoons in a pint glass of lukewarm watter every morning for 3 weeks. The exact amount depends on the brand you buy, but it should make a thick undrinkable jelly if you let it sit for 10 minutes. I drink it immediately after stirring it smooth, while it's still drinkable. I have used the psyllium mixture made by colonix past 5 or 6 times. I liked it very much, because they make it taste nice and it's good quality too, but you can't get in the UK anymore, so this morning I ordered the first one from Amazon. Beware, the first days I usually get pretty bad tummy cramps.
2.) Water: you need to drink a lot. Another pint just after the pint of psyllium. And at least another 1.5l of good quality bottled water (or tap, although tap water in my neighborhood in Liverpool sucks).
3.) Laxatives: I usually drink a laxative herbal tea in small amounts. A mug a day in the morning, steeped only for a minute or two. If I follow the recommended dose which is usually higher, my tummy hurts too much. Too much stuff to pass too quickly through one's stomach.
Do that for three weeks followed by one week of rest. That usually does the trick for me, although the first time I did it I had to go on for like two months. Nowadays I start feeling better after a week or so.
It is advisable to eat very simple during the cleanse. This helps in avoiding foods that got you in the trouble in the first place, although identifying the one evil thing is very hard. As someone here already pointed out, this may be a consequence of food alergies. The usual suspects are dairy and gluten, although it can be way more complicated than that. My sister and her kids have it worse than me, they tested positive to dozen of different very common foods. For me, it is dairy I believe: for many years I have been avoiding cow's milk completely and ate only sheep and goats cheese. I met a british girl recently and she drinks loads of milk. She told me about this lactose free milk and I started having it with my cereal in the morning instead of my usual rice drink. This is the only change in my diet since many months and so I am guessing this is the thing that caused my most recent episode. It must not be lactose in the milk that I am intolerant to but something else, I am just reading up on that.
Recently I have also noticed that ibuprofen is really bad for my tummy. I haven't taken much of before settling down in the UK, but it seems to be the first choice pain killer here. Or at least that's what my GP told me to take the last two times I complained about my back ache (yep, I am a real catch, don't tell my new girlfriend...). If you are taking any at this moment, consult your GP -- there are more expensive alternatives that are much more gentle with my tummy which my French GP was prescribing to me (I don't remember the name, but I can find it back if you need it, I googled it and it should be available also in the UK).
Concerning the charcoal tablets: I have been taking those my whole childhood and they work for me if I drink plenty of water. Otherwise they just stop all the traffic down there. They work only short term though. My problems are back like a week after taking them.
I will start as soon as my psyllium arrives and will get back to you in a month or so to let you know whether my routne still works for me or whether I am back to being broken, this time for good (I am really scared, I am usually a relatively happy person but this kind of discomfort just makes me cranky and depressed all the time).