Hi Roy, sorry to come to this discussion late in the day. Lots of useful posts being made which I have been reading.
Just before I went up to Lewis I printed off this article http://www.soue.org.uk/souenews/issue8/thom.html
and did a bit of reading up on Alexander Thom and Megalithic Astronomy. It's an interesting article which also touches on the 'megalithic yard' - a linear measurement of 0.829m or 2.72ft.