The submarine used was NOT Varangian! 'It' was in fact two boats, P614 and P615, both built for Turkey by Vickers Armstorng at Barrow-in-Furness but kept hold of by the Royal Navy for the duration of the war. P615 was sunk but P614 was eventually delivered in 1945.

The confusion no doubt arose because someone looked up P61 (as I did) and found Varangian! When in fact, the last digit of both P614 and P615 was in fact just painted out....

There are some extremely realistic moments in the film. These Turkish boats were very similar to the S-class. As no S-class submarines survive, the shots of them (as P61) and of the depot ship "Forth" form part of an interesting record now, as well as an excellent film.