HMRC have the following advice for contractors who expect their contract to be for a term greater than 2 years:

http://www.hmrc.gov.uk/manuals/nimmanual/nim06270.htm

Basically what the guidance means is that as soon as a contractor knows they will be working at the same place for a period of greater than 2 years then (because the workplace can not then be classed as temporary) they cannot from that date claim travel expenses (unless spend less than 40% of their time there i.e. works from home also).