http://en.wikipedia.org/wiki/Alto_clarinet
"The range of the alto clarinet is from the concert G or G♭ in the second octave below middle C (i.e. bottom line of the bass clef) to the middle of the second octave above middle C, with the exact upper end of the range depending on the skill of the player"
Alto clarinets are pretty much obsolete, except for military band use and have been superseded by the basset horn -
http://en.wikipedia.org/wiki/Basset_horn which is written in F rather than Eb, but has a few extra low notes that the alto clarinet doesn't have.
How on earth marching band musicians make themselves heard on the alto clarinet, I've no idea. Mine is quite quiet, with a mellow sweet tone, a sod to play, but excellent for improving the embouchure.
For anyone intent on getting into low clarinets, try one or two out before you buy - they're not easy..
I don't know what those Gear4music alto and bass clarinets are like, £700 ish is a bit much for my budget. I quite fancy one of their £99 Eb clarinets though..
A decent mouthpiece helps a lot. Vandoren are no longer as cheap as they were when I got one for my alto, but it made life a lot easier. Charles Bay makes great mouthpieces, but I don't want to look at the price.
I use alto sax reeds on the alto clarinet, they're easier to play, although purists would tell me they ruin your tone...
I got my 1949 Leblanc alto clarinet from this gentleman -
http://www.lynsgarden.co.uk/Rosecroft/Clarinets.html
he no longer restores and sells instruments, but his website has some useful info