This post is for smartphone owners only.
Do not rely upon your phone for mapping. Your phone will fail when out of battery, when wet and when cold... Take a paper backup with you an all trips to the hills.
Links are all desktop - the apps are all available in your App Store. All come highly recommended so get them!
viewranger - https://www.viewranger.com/en-gb - my go to app - why? because it works, it has good customer support and it does the stuff that strata or wikiloks does BUT with OS Maps as a paid for option.
maps.me - https://maps.me - my go to stripped down map app. Not cluttered, easy to read and follow and FREE!
OS Maps - https://www.ordnancesurvey.co.uk/shop/os-maps-online.html - The professionals choice but now viewranger have this at the same price it seems a no brainer to go there...
strava - https://www.strava.com - The runners and cyclists choice - used by Martin :)
wikiloc - https://www.wikiloc.com - The Walkers choice - used by Al