I didn't know much about Neame other than The Poseidon Adventure, which clearly is not representative of the rest of his work. But I enjoyed both of the films, although they were completely different from each other.
Tunes of Glory is a very well-acted drama about a Scottish regiment, with Guinness and John Mills on top of their game (they actually were offered the opposite roles from each other and decided to switch). It's dialogue-heavy but subtly gripping. The Horse's Mouth is essentially a comedy, although I think it's quite a wise portrayal of the life of a painter (albeit, a loony one) in general. It's much more lightweight than Tunes, but I enjoyed it all the same. Watching the two in short succession really showed off the versatility of Sir Guinness. So yeah, if you're at all interested, I think they're both worth the time.