I remembered this as a poor film
And watching it again many years later I was right to do so. I think 3 actors manage to turn in a decent performance; Robert Shaw, Edward Fox & Michael Byrne. Everyone else is overcome by the clunky script with its awkward expositions and a series of tensionless moments - even the music is annoying.
It's a long time since I read the book but the main thing I remember is how disappointing it was compared to The Guns Of Navarone. I suspect for this film they took it's not particularly good story, 'improved it' (sic) with elements from Where Eagles Dare and hoped they would catch a little of the first films success - they failed!