Follow the B4393 from Llanfyllin until you reach Lake Vyrnwy.
Cross the dam and turn left. Go past the first car park and retail unit and continue for 200m. The Old Barn Cafe is on the right, in the courtyard.
There is ample room to park further down the hill, towards the children's play area.
For the Satnav users, just set the post code to SY10 0LZ