It's free to download as a pdf if you're a member of the Prehistoric Society:
http://journals.cambridge.org/action/displayIssue?decade=1930&jid=PPR&seriesId=1&volumeId=4&issueId=01&iid=9261676
that link has expired, i found it via https://www.cambridge.org/core/journals/proceedings-of-the-prehistoric-society/article/abs/excavation-of-an-untouched-chamber-in-the-lanhill-long-barrow/B6EDD43073D53C645A1A70267D4B66CA
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