I've found something quite good: Colon Cleanse from Biovea (an online shop I've been buying from for years so I know it's trustworthy).
Colon Cleanse is made from herbs and fruit extracts and you just add between 30 and 50 drops to a little water once or twice a day. Despite being a murky dark brown colour, it doesn't really taste of anything. A bit on the pricey side but it works.