According the label on a bottle - HP sauce is 'the only and original brown sauce made since 1899' and its distinctive taste is the 'result of HP's dedication to sourcing the highest quality ingredients and using aclosely guarded secret recipe.'
It also says that it contains no artificial colours, flavours or preservatives and is low in fats and suitable for vegetarians.
Ingredients listed include: malt vinegar (from barley), tomatoes, molasses, spirit vinegar, dates, glucose syrup (from wheat), sugar, salt, modified starch, rye flour, tamarind extract, spices, onion extract.
I think it is brilliant and would never be without a bottle but be careful I had some in Canada once and it was made under licence in Ottawa, I recall, but didn't taste like the original.
If you want to know more check out www.hpfoods.com
hope this helps