The negotiations were held at an Uxbridge inn now called the Crown and Treaty house. Above the door is written:
‘Ancient Treaty House where the ill-fated Charles I held the memorable, but unsuccessful treaty with his Parliament in January 1645’. When I lived in Uxbridge in the 1970's, this was the only pub in town selling real ale.
More at http://www.olivercromwell.org/Uxbridge.htm
Uxbridge is in the upper-left corner of this J. Cary map
Chart showing the number of references in each month of the diary’s entries.