In reply to FesteringSore:
I've been using ViewRanger for over 6 years and find it indispensable.
It's been great for hiking, scrambing, road cycling and when drive, e.g. when trying get to a new crag. I've bought loads of OS Maps at 1:50,000 and 1:25,000 over the years. It's easy to use because I've been using OS mapping since I was a kid.
Strain on the phone battery has improved massively over the years. That was previously a constraint but isn't anymore. On multi-day camping / hiking trips and do need to manage power, so I use an external (Brunton) battery. I ensure I've fully charged before setting off on each hike.
Only downsides are that they sometimes 'update' it with updates I don't need and that can mean I have to re-learn the app on occasion. Also, I prefer the old app icon.