Guy's character changes in rank in various scenes. for example he is a Lieutenant when sent on his second two days leave, when he returns early (after falling out with his ex wife) he's on the train clearly wearing the pips of a captain. Then reverts back to being a Lieutenant, until he's a Captain again when off the coast of Africa, reverting to a Lieutenant again when posted back to London (possibly a demotion there), but in the next scene on arrival in Scotland, he's once again a Captain.