Hi Fiona
Coincidentally I got a Boots magazine through the post today and I see they are advertising a product called Detoxil which contains Artichoke, Dandelion, Grapefruit Extract and other good stuff. There is a website - www.detoxil.com.
I forgot to mention that Neways also do a Colon Cleansing Tea called Puri-Tea, which they suggest you use over the weekend because of the "side effects" which presumably are lots of trips to the loo! Haven't tried it though.
Petal x