I just thought I'd let you all know I've found the most fantastic bath support to date!
I've got three little ones and found with my first two that the bath supports I bought them once they could sit were really not very comfortable.
Both babies were prone to slipping backwards in the ring stype seats and this resulted in rather painful bumps to their private parts. I felt the plastic supports should have been softer and more padded to prevent this happening so I gave up using them after one of two times.
However with baby number three I've found something fab!! Mothercare do a support called an Aquapod. Its soft enough not to harm little bits and is great for little ones who like to be very active in the tub! My little man loves to splash quite violently and used to tip himself backwards and slip over all the time. Now he can sit on the nice rubbery mat, with the soft support behind his back and between his legs and splash to his hearts content. Leaving my hands free to shampoo his hair etc.
Quite pricey at about £17.00 but well worth the money especially if you have more than one baby as the mat is long enough to fit another child on the end.