The best kind to use is Caltrate 600 w/ vitamin D. This is the right kind of calcium for drying you up and doesn't have too much magensium (which is needed to help you absorb the calcium into your bones, but which counteracts the constipating effect). I don't know what name that brand might go under in the U.K., but you can always order from an online pharmacy if you have to. The recommended dosage (for use as a supplement) is 2 pills. I think you can take up to 3 a day, but make sure you ask your chemist because I have read that you can harm your kidneys by taking more than a certain amount reguarly.
You can also try the probiotics to see if they help any. If you have a bad bacteria in your guts it can help push it out, though it can take a long time because they're just good bacteria, not a medicine. I liken the situation with bacteria in the intestines to housing: bad bacteria is like an urban blight and you can eventually push it out with good bacteria coming in and cleaning up the neighborhood, but it does take time. In the U.S. there are probiotic products like Digestive Advantage and Culturelle. You may find something similar in the U.K. or you can at least eat a yogurt a day and/or take acidophilus pills (they can be found where herbal remedies are sold).
Definitely get your stools checked for stuff because you can have all kinds of nasty things living in your intestines that won't show up in your urine or blood.
And my theory for dealing with family members who are in denial is to get a good toliet full and not flush. Then either let people in the house discover your diarrhea poops on their own (and thus realize that something's wrong with you) or drag them in there, point to the toliet and ask, "Does that look normal to you? If my gut problems are all in my head, why do they appear to be in the toliet as well?" I'm all for being agressive, lol. They're being rude about not believing you, so I'm all for being rude around them, like not flushing and passing gas in their company. Sometimes it takes something rude to make people think objectively.