In reply to HardenClimber:
Interesting. I'm surprised it's as much as 50 metres.
I started using OS Locate for altitude after noticing that ViewRanger (or at least, the version on my 10-year-old phone) is often way out when on summits in the Highlands. My guess is that this due to calculating altitude from a digital elevation model, with insufficient resolution to handle the case where one is close to a steep drop.
From memory, OS Locate seemed to give a value within 2 or 3 metres of the summit height read from the map.