In reply to andyd1970:
Harveys, sometimes with OS sheets as back up.
I'm still using 1:40k Harvey maps from when they first appeared on the general market; they've been folded and got a soaking from wind, rain and snow countless times but are still going strong.
The BMC series cover larger areas per sheet but aren't quite so easy to use because they are bigger and so don't fold as well. In general though, the detail is excellent for mountain navigation and the maps of the Cuillin are a masterpiece.
Downsides? My 15 times table is always rusty, and they could sometimes have more named features on them - then the OS series would be pretty much redundant for me.