There is more than one reason to see this but if for nothing else see it for Guinness's performance. The screenplay dies a bit when it is away from the POW camp & shortening those segments would have helped & it would not have diluted the overall message
This is another example of writer/director/leading man. Passchendaele makes an appearance after an hour but even then has to share screen time with the romantic leftovers from earlier.
The best compliment I can pay it is that it is still well worth watching 90 years on. The screenplay is very good and as it is played out whimsically it is also very entertaining.
Although the true story of the code breakers is now reasonably well known this dramatisation is well worth it considering the quality of the cast and the production's period details.