South East of Welshpool Andy IIRC.
Its not very well used.
When I first drove it 2 or 3 years ago it hadn't been driven for a long time and I had to clear a lot of fallen trees out of the way. The farmer was quite happy for us to drive it and said no one had driven it for years. It still doesn't look like it gets much use.
There are a lot more lanes in that area that I never got round to exploring, so there could be more, that might be even more challenging.