“does anyone know the state of the trail that loops round behind Tryfan? Worth a go?”
Festooned with Hikers heading up to Bristly Ridge on the west side, many staircases too. The east side is probably mostly doable.
Great rugged scenery – my suggestion is to ride/push up the east side and hide your bike near the Tryfan’s col, hike down the heather terrace, then scramble up little/central gully on Tryfan. Return to the col, climb bristly ridge and descent down the scree back to the col. Retrieve bike and bomb down – sounds like a good day!
I think you are brave tackling that cliff drop down east to the col from Llewelyn, with a bike. Winter hiking I’ve been turned back by that when I forgot my crampons – I just could not get my boots to grip the ice and snow covered rock face. Smashing my legs at the bottom and falling over the north side was not worth contemplating. I imagine the switchback trail from the col down to the reservoir is very serious on a bike too.