Below the Great Bass is

Contra Bass in F

There are others below that, but they are rare as yet, though there do seem to be one or two playing a C sub contrabass in the Society of Recorder Players.

I play an F contra bass. Wonderful instrument with a fabulous tone.

Rowan is right. The various C and F instruments are what are nearly always played, certainly among amateurs. Professionals (usually Conservatoire trained) do play instruments pitched in other keys, the most common being the Voice Flute which is a Tenor in D, a tone above the C tenor. It is quite widely used for playing Baroque flute music as the Baroque one key flute was also pitched in D.

Jack Campin is fairly unusual as an amateur playing a G alto and I can appreciate his reason for doing so, though he does seem to have a liking for unusual wind instruments. :-)