I bought the MemoryMap 1:50000 map of Wales not long ago. It's very good, very useful and although I don't use GPS, I think it will do what is being discussed.
One note of caution is that MemoryMap shows Public Rights of Way through Penrhyn Quarry that closed years ago. Other PROW's are also suspect....but this, I suspect, is Gwynedd County Council's fault for non-notification (as with the street name changes etc etc) There are some very "iffy" databases out there as a result!