Hi Mark
There are two shutter options:
1) 8 seconds max with bulb mode (i.e. you have to keep the button pressed down to keep the shutter open)
2) Timed release, max 10 minutes
Sounds like you're on 1)
To change...
In manual mode (M) press 'Menu' on the back, scroll down to 'Show all menus' and press the select button (the one in the middle of the up/down/left/right button). Scroll to 'Exposure options' press the select button, scroll to 'Bulb/Time', press the select button scroll to 'Timed release' press the select button and select from 30s, 1m, 3m, 5, 10m and press select according to preference.
I keep mine on 10 minutes, which gives you etc, etc, 2s, 4s, 8s then 10m on the shutter options controlled by the thumbwheel.
As the Colonel says, 8s is fine for most shots, and this method means you've got the 8s timed release as well as flexibility for much longer shots where you've got the shutter open for as long as you feel is right.
HTH
my orders are to sit here and watch the world go by