Ok, probably not as helpful as you'd like, but:
Best advice is to use a combination of Magic map, OS 25000 and the instructions in Ian McNeil Cooke's Mermaid to MerryMaid.
From memory: From Gurnard's Head, take the road which heads inland to Treen common. Approx 200yds from the coast road is a footpath on the right (SW439371). Climb the bank/stile for the footpath.
Head off South across the fields toward the farmhouse until you come to a brook (SW438364). Cross the brook via the footbridge and turn left. If you turn right, there's a well a short way NW of the bridge and a couple of fields away is the Borsporthennis Quoit.
From here, I recall we walked along the track for a short while, then headed right across the fields (this is farmland I believe, so may be best to ask, though I think there are stiles). This is where the McNeil Cooke book helps as he lays out the fields, gates and trackways. From my poor memory, it's about 4 or 5 fields down and 3 fields in. Moth/Jane can probably confirm this.
You are unlikely to spot the house unless you're in it - it looks just like part of the normal field boundary until you get there! We had to confirm our location via GPS (which didn't help in finding it 'cos I was new to GPS then :-)
Have a great time, we're there from 7th October. If anyone else is down there at that time, give me a shout and we'll arrange a meet.
