Although I've never been, intense research for the Challenge (i.e. Googling) as I'm passing that way myself makes me think it's in the village at NO491798, by the church.

Google Maps Street View

St Drostan's is the Scottish Episcopal Church in Tarfside village, marked on the map as a cross; the church with the tower further east is the Church of Scotland. The hostel building is adjacent to the church, a few steps north along the minor road to Milton. Being pernickety I would reckon the grid ref as NO490798. Be sure to drop in for tea, scones, sympathy, food, bed, breakfast, hot shower or just the craic.

If you miss the building there is a danger that you could end up in the hospitable Mason's, from which it is almost impossible to escape with dignity intact. There may be singing.

I knew it would be easier to ask you lot...thanks for your help