I have always beleived that in many constituencies Tories could comfortably vote for Plaid candidates as a safe bet against Labour. I have no doubt that (Lord) Dafydd Wigley's big majorities were based on an appeal across Plaid/Tory boundaries with, I will acknowledge, some Labour supporters thrown in for good measure.
As I have argued before it is possible that this cozy relationship is more strained after the One Wales agreement. This is one element of the additional interest in the Westminster election when it arrives. Welsh Tories clearly see some potential ground to exploit. With Plaid's grip on fortress Gwynedd facing a challenge on several fronts it will be an interesting few weeks/months/years.