Thanks for your replies.
I've used OS maps a few times with the downloaded digital versions of paper maps I've purchased, it seems ok but I was looking for an app that can also strip it back to the basic maps (preferably for free).
On the subject of altitude and other information, I'm not sure if you've seen it but there is an "OS locate" app that provides a compass, altitude and coordinates
I've taken a look at TopoGPS before but for the life of me I can't find any stripped back free maps. Is there a function like that hidden in it?
I must say, Locus looks pretty promising at first glance (he says after spending an hour exploring it )
How reliable is it when recording a track over 5 or 6 hours?
Also, what is its battery usage like?