Wow! photos do not do this stone justice..it's a big un!
Easy to find just up the drive to Trelew Farm. It is on their land but they are happy for people to visit. In fact guests using the farm for B&B are encouraged to walk through the fields to visit Boscawen-un passing another menhir on the way.
See link for details http://www.trelew.co.uk/
This is off the B3283, 2km North East of St Buryan. A track to Trelew B&B is just after the brow of the hill, almost forming a crossroads with a very minor farm track. The menhir is not far from the junction. At the first 'lay-by' on the left two field entrances meet. The menhir is then 30 metres from one of these gates. I'm not sure whose land this belongs to. The field bore the aftermath of a maize crop, so I would imagine that in summer the stone may almost be swamped by maize! The stone measures 250cm (H) x 110 (W) and is quartzy and very impressive, although very lonely!
The site was excavated in 1871 by W.C.Borlase (busy year!) and cremated remains, wood, flint, clay and calcified bone were found in a pit 3ft below ground level, close to the foot of the stone. The stone appears to be unusually erected on its narrowest end.