Their are a number of large and professional tour companies in Tunisia.
Some of the smaller ones offer great value and may be a good bet for the more adventourous however their are also some with offices in the uk like wigmore travel which offer you abta protection et al
Have a look at the Tunisian tourist board website
www.cometotunisia.co.uk (they make it more like our site everytime I see it :-) ) for a list of officially recommended tour operators.