Thanks for your advice, oldcpu.

I've had some success following the links you posted- I've identified the board type and codec, which is Intel ALC883/888.
I tried the steps mentioned in the guide here for the Intel HDA chipset, and after specifying various models I have control over both laptop speakers and headphone levels independently.
The channels in kmix don't appear to be labelled correctly, but having run out of time for now, and having managed to resolve the problem of using headphones with the speakers OFF I have decided to leave it like this for the time being.

The only problem now is that the "surround" volume is controlling the speaker volumes, and the "front" volume the headphones. As it's all working I'm not too fussed at this stage- there's nothing else wrong that I've noticed.
If I improve the situation with any further fiddling I'll post back, but my thankns in the meantime