Hi Nigel, you've probably already seen this link. Posting it here for its fascinating account of the history of the use of sarsens and the ingenious manner in which it was broken up and 'harvested'. Thankfully it all stopped when concrete started to be manufactured in the early part of the 20th century.
http://www.stonehengemonument.co.uk/2017/02/the-sarsens-of-west-woods.html?fbclid=IwAR3sDzjoFI__8UYUgnFAd_7POKWONjF0NMK5_trxbxJC722y5XGx2G0IgPg