Thanks for the input so far.
Fortunately my job involves a moderate amount of physical activity and I regularly practice Taiji in the mornings and evenings. This probably counteracts some of the effects of sitting for long periods.
I would be interested to hear of other peoples experiences of discomfort when sitting and how they deal with it.
I can recall from the first retreat I went on, Mr Goenkas' challenge to sit with "fierce determination" for one hour, and not to be distracted by physical discomforts when sitting. Followed by agonising sessions, and then sometimes a kind of trancendence of pain. Sometimes one could barely walk afterward, other times I could have leapt up and done a little dance. At home it can be fairly similar, but one feels less inclined to push oneself as hard.
Given the emphasis on sitting meditation within the Buddhist tradition I'm surprised so little is said about it, the history, the reasons for the various postures, the hands, the appropriate way to sit. Just the basic physical mechanical aspect. 2500 years experience should yield something.
Where's the manual on best practice?
The Zen Tradition of course takes it quite seriously I understand.