Hi Jayne,
If you'd care to read my report on this section, titled Lake Vrynwy, which is on page six and dated May 22nd 2009 it will tell you where I saw the Pied Fly and Wood Warbler plus lots of other stuff. The lake reserve isn't too far from Bala. When I was there I'd been somewhere and came via Bala and over the Hirnant Pass [very scary in fog] but there is a more sensible road to take
If I remember it's about 15 miles from Bala car park to the lake.
Although I've been to Anglesey I didn't see your requested butterfly.
If you want any more info send a PM.
Hope this helps.
Cheers
John